Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 131,354	$ 145,610
Specific valuation allowance allocations	(23,410)	$ (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	365		$ 0
Specific valuation allowance allocations	(64)		0
Loans receivable, fair value disclosure	301		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	56,788		24,171
Specific valuation allowance allocations	(1,520)		(1,340)
Loans receivable, fair value disclosure	$ 55,268		$ 22,831