Off-Balance Sheet Arrangements, Guarantees and Other Commitments - Summary of Commercial and Standby Letters of Credit (Additional Information) (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Commitments and Contingencies Disclosure [Line Items]
Deferred fees	$ 734	$ 736	$ 27,057
Standby Letter of Credit
Commitments and Contingencies Disclosure [Line Items]
Deferred fees	11,000		$ 12,000
Collateral in the form of cash	$ 963,000
Consolidated venture capital and private equity fund investments
Commitments and Contingencies Disclosure [Line Items]
Commitments to invest, period from the inception of the fund	10 years
Call unrestricted, percentage of committed capital	100.00%
Consolidated venture capital and private equity fund investments | Lower Limit
Commitments and Contingencies Disclosure [Line Items]
Expected commitment period to invest in venture capital and private equity funds (in years)	5 years
Consolidated venture capital and private equity fund investments | Upper Limit
Commitments and Contingencies Disclosure [Line Items]
Expected commitment period to invest in venture capital and private equity funds (in years)	7 years