Fair Value Measurements (Impaired Loans Remeasured and Reported at Fair Value of Underlying Collateral) (Detail) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	$ 145,610	$ 94,023	$ 77,478
Specific valuation allowance allocations	(20,820)	(9,186)	(4,378)
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Carrying value of impaired loans before allocations	75,435	33,626	14,921
Specific valuation allowance allocations	(19,533)	(3,961)	(2,765)
Fair value	$ 55,902	$ 29,665	$ 12,156