Annual report pursuant to Section 13 and 15(d)

Stock Compensation - Additional Information (Detail)

v2.4.0.6
Stock Compensation - Additional Information (Detail) (USD $)
In Millions, except Share data, unless otherwise specified
12 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Apr. 16, 2007
Jul. 01, 1998
Apr. 16, 2007
Employee Stock Purchase Plan 1998 [Member]
Jul. 01, 1998
Employee Stock Purchase Plan 1998 [Member]
Dec. 31, 2012
CLASS A COMMON STOCK [Member]
Dec. 31, 2011
CLASS A COMMON STOCK [Member]
Dec. 31, 2010
CLASS A COMMON STOCK [Member]
May 31, 2003
CLASS A COMMON STOCK [Member]
Jun. 30, 2001
CLASS A COMMON STOCK [Member]
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]                        
Number of shares authorized for issuance                     11,215,154 8,215,154
Vesting period stock option awards 3 years                      
Expiration period of stock option awards 10 years                      
Number of shares reserved for issuance       7,500,000   3,000,000 2,781,415          
Intrinsic value of options exercised $ 1.9 $ 1.2 $ 20.9                  
Stock options granted 0 0 0                  
Number of shares remain available for grant 4,868,881                      
Unrecognized compensation cost related to unvested stock 4.7 11.3                    
Weighted average period for recognition of cost 3 years 8 months 12 days 10 months 24 days                    
Total fair value of shares vested 13.1 13.9                    
Period utilized to implement employee stock purchase plan 6 months                      
Interval for purchases occurring 6 months                      
Eligibility to Participate Stock Purchase Plan At least 20 hours per week and more than five months in any calendar year                      
Maximum percentage of employee's compensation to purchase common stock       15.00% 15.00%              
Percentage of price of common stock purchased       85.00% 85.00%              
Proceeds from issuance of common stock under the employee stock purchase plan, Shares 186,199 178,189 103,430         186,000 178,000 103,000    
Proceeds from issuance of common stock under the employee stock purchase plan $ 2.4 $ 2.0 $ 2.1