Fair Values of Assets and Liabilities (Summary of Assets Measured at Fair Value on a Nonrecurring Basis) (Detail) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	$ 120,302	$ 120,302	$ 105,088	$ 79,318
Fair Value Losses Recognized, Impaired loans—collateral based	3,288	7,174
Fair Value Losses Recognized, Other real estate owned	1,511	3,669
Fair Value Losses Recognized, Total	4,799	10,843
Measured at fair value on a non-recurring basis:
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	67,700	67,700
Other real estate owned	35,331	35,331
Total	103,031	103,031
Measured at fair value on a non-recurring basis: | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	0	0
Other real estate owned	0	0
Total	0	0
Measured at fair value on a non-recurring basis: | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	0	0
Other real estate owned	0	0
Total	0	0
Measured at fair value on a non-recurring basis: | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Impaired loans—collateral based	67,700	67,700
Other real estate owned	35,331	35,331
Total	$ 103,031	$ 103,031