Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 62,154	$ 12,224
Held-to-maturity, Amortized Cost	729,088	607,833
Held-to-maturity, Estimated Fair Value	724,250	598,119
Total, Amortized Cost	791,453	620,375
Total, Gross Unrealized Gains	3,552	2,681
Total, Gross Unrealized Losses	(8,601)	(12,748)
Total, Estimated Fair Value	786,404	610,308
Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	729,088	607,833
Held-to-maturity, Gross Unrealized Gains	3,449	2,681
Held-to-maturity, Gross Unrealized Losses	(8,287)	(12,430)
Held-to-maturity, Estimated Fair Value	724,250	598,084
Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	213,989	144,950
Held-to-maturity, Gross Unrealized Gains	969	164
Held-to-maturity, Gross Unrealized Losses	(4,405)	(7,085)
Held-to-maturity, Estimated Fair Value	210,553	138,029
Held-to-Maturity Securities | Investment Securities | Other investments
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	8,861	8,778
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(191)	(228)
Held-to-maturity, Estimated Fair Value	8,670	8,550
U.S. Agency Obligations | Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	14,964	19,960
Held-to-maturity, Gross Unrealized Gains	67	69
Held-to-maturity, Gross Unrealized Losses		(856)
Held-to-maturity, Estimated Fair Value	15,031	20,029
U.S. Agency Obligations | Investment Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	62,365	12,542
Available-for-sale, Gross Unrealized Gains	103	0
Available-for-sale, Gross Unrealized Losses	(314)	(318)
Available-for-sale, Estimated Fair Value	62,154	12,224
State and Municipal Obligations | Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	114,123	39,155
Held-to-maturity, Gross Unrealized Gains	577	10
Held-to-maturity, Gross Unrealized Losses	(402)	(856)
Held-to-maturity, Estimated Fair Value	114,298	38,309
Corporate Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	55,000
Held-to-maturity, Estimated Fair Value	51,219
Corporate Debt Securities | Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	76,041	77,057
Held-to-maturity, Gross Unrealized Gains	325	85
Held-to-maturity, Gross Unrealized Losses	(3,812)	(6,001)
Held-to-maturity, Estimated Fair Value	72,554	71,141
Mortgage-Backed Securities | Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	515,099	462,883
Held-to-maturity, Gross Unrealized Gains	2,480	2,517
Held-to-maturity, Gross Unrealized Losses	(3,882)	(5,345)
Held-to-maturity, Estimated Fair Value	513,697	460,055
Mortgage-Backed Securities | Held-to-Maturity Securities | FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	178,287	144,016
Held-to-maturity, Gross Unrealized Gains	351	195
Held-to-maturity, Gross Unrealized Losses	(1,752)	(2,457)
Held-to-maturity, Estimated Fair Value	176,886	141,754
Mortgage-Backed Securities | Held-to-Maturity Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	245,469	217,445
Held-to-maturity, Gross Unrealized Gains	1,903	2,175
Held-to-maturity, Gross Unrealized Losses	(1,737)	(2,524)
Held-to-maturity, Estimated Fair Value	245,635	217,096
Mortgage-Backed Securities | Held-to-Maturity Securities | GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	84,091	92,475
Held-to-maturity, Gross Unrealized Gains	147	119
Held-to-maturity, Gross Unrealized Losses	(393)	(364)
Held-to-maturity, Estimated Fair Value	83,845	92,230
Mortgage-Backed Securities | Held-to-Maturity Securities | SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	7,252	8,947
Held-to-maturity, Gross Unrealized Gains	79	28
Held-to-maturity, Estimated Fair Value	$ 7,331	$ 8,975