Fair Value Measurements - Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017	Sep. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 78,356	$ 145,610
Specific valuation allowance allocations	(21,488)	$ (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	12,876		$ 0
Specific valuation allowance allocations	(2,599)		0
Loans receivable, fair value disclosure	10,277		0
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	28,638		64,287
Specific valuation allowance reversal of prior allocation	4,256
Specific valuation allowance allocations			(13,477)
Loans receivable, fair value disclosure	$ 32,894		$ 50,810