Fair Values of Assets and Liabilities - Schedule of Changes in Level 3 Assets and Liabilities Measured at Fair Value on Recurring Basis (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	$ 18,094	$ 36,751	$ 27,459	$ 35,892
Total gains or losses included in earnings	5,262	0	8,102	0
Total gains or losses included in other comprehensive income	(4,647)	(87)	(118)	410
Purchases	5	137	154	547
Issuances	0	0	0	0
Sales	(12,289)	0	(12,289)	0
Settlements	(2,458)	(6)	(19,341)	(54)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Balance, end of period	3,967	36,795	3,967	36,795
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Pooled Trust Preferred Collateralized Debt Obligations [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	14,132	33,741	23,646	33,292
Total gains or losses included in earnings	5,262	0	8,102	0
Total gains or losses included in other comprehensive income	(4,647)	(87)	(118)	410
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	(12,289)	0	(12,289)	0
Settlements	(2,458)	(6)	(19,341)	(54)
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Balance, end of period	0	33,648	0	33,648
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Equity Securities [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	1,670	1,670	1,670	1,670
Total gains or losses included in earnings	0	0	0	0
Total gains or losses included in other comprehensive income	0	0	0	0
Purchases	0	0	0	0
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Balance, end of period	1,670	1,670	1,670	1,670
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	0	0	0	0
Other Assets [Member]
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance, beginning of period	2,292	1,340	2,143	930
Total gains or losses included in earnings	0	0	0	0
Total gains or losses included in other comprehensive income	0	0	0	0
Purchases	5	137	154	547
Issuances	0	0	0	0
Sales	0	0	0	0
Settlements	0	0	0	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3	0	0	0	0
Balance, end of period	2,297	1,477	2,297	1,477
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3	$ 0	$ 0	$ 0