Short-Term Borrowing - Additional Information (Detail)		12 Months Ended
	Oct. 08, 2015 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Short-term Debt [Line Items]
Short-term borrowings		$ 209,871,000	$ 173,397,000
Line of Credit Facility, Maximum Borrowing Capacity		$ 320,000,000
Number Of Unsecured Bank Credit Facilities		4
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		1.43%	1.30%
Line of Credit Facility, Commitment Fee Amount		$ 144,810,000	$ 106,000
Ratio of Indebtedness to Net Capital		0.65
Uncommitted Line Of Credit Facility One [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 150,000,000
Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Line of Credit Facility, Expiration Period	5 years
Line of Credit Facility, Maximum Borrowing Capacity		$ 275,000,000
Committed Line of Credit Facility Two [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis		LIBOR rate, plus 1.00 percent
Line of Credit Facility, Maximum Borrowing Capacity		$ 30,000,000
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
Committed Line of Credit Facility Three [Member]
Short-term Debt [Line Items]
Debt Instrument, Description of Variable Rate Basis		Lender's base rate, plus 0.85 percent
Line of Credit Facility, Maximum Borrowing Capacity		$ 35,000,000