Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 115,097	$ 145,610
Specific valuation allowance allocations	(24,662)	$ (20,820)
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	14,359		$ 0
Specific valuation allowance allocations	(799)		0
Loans receivable, fair value disclosure	13,560		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	52,048		21,686
Specific valuation allowance allocations	(1,149)		(561)
Loans receivable, fair value disclosure	$ 50,899		$ 21,125