Short-Term Borrowing - Additional Information (Detail)		12 Months Ended
	Oct. 08, 2015 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Feb. 29, 2016 USD ($)
Short-term Debt [Line Items]
Number Of Financial Institutions With Bank Credit Facilities		3
Line Of Credit Facility Maximum Borrowing Committed Capacity		$ 170,000,000
ParticipatingLendersInTheRevolver		5
Short-term Debt, Weighted Average Interest Rate		1.30%	1.15%
Line of Credit Facility, Commitment Fee Amount		$ 106,010,000	$ 87,000
Short-term borrowings		173,397,000	$ 88,231,000
Line of Credit Facility, Maximum Borrowing Capacity		$ 320,000,000
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		4
Number Of Unsecured Committed Facilities		2
Committed Credit Facility Capacity Amount Expired		$ 40,000,000
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
Uncommitted Line Of Credit Facility One [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		150,000,000
Line of Credit Facility, Current Borrowing Capacity	$ 150,000,000
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 200,000,000
Line of Credit Facility, Expiration Period	5 years
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
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 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
Subsequent Event [Member] | Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity				$ 275,000,000