Commitments, Contingencies and Guarantees - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Commitments and Contingencies Disclosure [Line Items]
Notional amount outstanding	$ 0	$ 0
Average notional amount of open futures contract	0	400,000
Gain (losses) from net futures activity	(6,000)	(142,000)	$ 35,000
Average of contracts to purchase and to sell foreign currency	36,800,000	40,500,000
Net gains on foreign exchange contracts	$ 1,900,000	1,600,000	$ 1,800,000
Maximum [Member]
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit term	6 years
Standby letters of credit	$ 316,100,000	376,600,000
Minimum [Member]
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit term	3 months
Related Parties [Member]
Commitments and Contingencies Disclosure [Line Items]
Standby letters of credit	$ 42,500,000	$ 67,400,000