Quarterly report pursuant to Section 13 or 15(d)

Line of Credit, Short-Term and Long-Term Borrowings - Additional Information (Detail)

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Line of Credit, Short-Term and Long-Term Borrowings - Additional Information (Detail) - USD ($)
 6 Months Ended
Jun. 30, 2015
Dec. 31, 2014
Line Of Credit Facility [Line Items]    
Outstanding letters of credit $ 4,400,000 $ 3,400,000
Short-term borrowings 1,340,000 $ 1,810,000
Maximum amount of credit facility 250,000,000  
Increase in maximum amount of credit facility $ 100,000,000  
Maturity date of credit agreement Jun. 30, 2020  
Line of credit facility, interest rate Borrowings bear interest at our election based on (a) LIBOR or (b) the greater of (i) the Prime Rate, (ii) the Federal Funds Rate plus 0.5% and (iii) LIBOR for a 30-day period plus 1.0%, in each case, plus an applicable margin based on the average daily principal balance of revolving loans available under the Credit Agreement.  
Unused line of credit fee 0.25%  
Maximum limit for letters of credit $ 100,000,000  
Debt closing and arrangement fees $ 1,100,000  
Line of credit facility, expiration period 5 years  
Line of credit facility, outstanding amount $ 1,300,000  
Federal Funds Rate [Member]    
Line Of Credit Facility [Line Items]    
Line of credit facility, interest rate Federal Funds Rate plus 0.5%  
Interest rate of line of credit agreement 0.50%  
LIBOR Loans [Member]    
Line Of Credit Facility [Line Items]    
Line of credit facility, interest rate LIBOR for a 30-day period plus 1.0%  
Interest rate of line of credit agreement 1.00%