Short-Term Borrowing - Additional Information (Detail) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Short-term Debt [Line Items]
Number Of Financial Institutions With Bank Credit Facilities	3
Letters of Credit Outstanding, Amount	$ 4,400,000
Short-term Debt, Weighted Average Interest Rate	1.15%	1.26%
Line of Credit Facility, Commitment Fee Amount	87,000	56,000
Short-term borrowings	88,231,000	105,666,000
Line of Credit Facility, Maximum Borrowing Capacity	210,000,000
Ratio of Indebtedness to Net Capital	0.65
Fixed Charge Coverage Ratio Minimum	1.20
Fixed Charge Coverage Ratio Maximum	1.0
Number Of Unsecured Bank Credit Facilities	6
Committed Line of Credit Facility Two [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis	Base rate plus 1.25 percent
Line of Credit Facility, Maximum Borrowing Capacity	30,000,000
Committed Line of Credit Facility Three [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR rate, plus 0.85 percent
Line of Credit Facility, Maximum Borrowing Capacity	35,000,000
Short-term Revolving Line of Credit Facility [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis	Lender's base rate plus 80 basis points
Line of Credit Facility, Maximum Borrowing Capacity	50,000,000
Line of Credit Facility, Increase (Decrease), Net	10,000,000
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	200,000,000
Committed Line of Credit Facility One [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis	LIBOR rate, plus 1.25 percent
Line of Credit Facility, Maximum Borrowing Capacity	$ 55,000,000
Base Rate [Member] | Committed Line of Credit Facility Two [Member]
Short-term Debt [Line Items]
Debt Instrument, Basis Spread on Variable Rate	125.00%
Base Rate [Member] | Short-term Revolving Line of Credit Facility [Member]
Short-term Debt [Line Items]
Debt Instrument, Basis Spread on Variable Rate	80.00%