Securities - Amortized Cost and Estimated Fair Value of Securities Available-for-Sale and Held-to-Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Securities Financing Transaction [Line Items]
Available-for-sale, Estimated Fair Value	$ 67,133	$ 12,224
Held-to-maturity, Amortized Cost	751,163	607,833
Held-to-maturity, Estimated Fair Value	746,497	598,119
Total, Amortized Cost	818,530	620,375
Total, Gross Unrealized Gains	3,596	2,681
Total, Gross Unrealized Losses	(8,496)	(12,748)
Total, Estimated Fair Value	813,630	610,308
Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	751,163	607,833
Held-to-maturity, Gross Unrealized Gains	3,508	2,681
Held-to-maturity, Gross Unrealized Losses	(8,174)	(12,430)
Held-to-maturity, Estimated Fair Value	746,497	598,084
Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	242,070	144,950
Held-to-maturity, Gross Unrealized Gains	1,012	164
Held-to-maturity, Gross Unrealized Losses	(4,521)	(7,085)
Held-to-maturity, Estimated Fair Value	238,561	138,029
Held-to-Maturity Securities | Investment Securities | Other investments
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	8,903	8,778
Held-to-maturity, Gross Unrealized Gains	0	0
Held-to-maturity, Gross Unrealized Losses	(189)	(228)
Held-to-maturity, Estimated Fair Value	8,714	8,550
U.S. Agency Obligations | Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	14,966	19,960
Held-to-maturity, Gross Unrealized Gains	41	69
Held-to-maturity, Gross Unrealized Losses		(856)
Held-to-maturity, Estimated Fair Value	15,007	20,029
U.S. Agency Obligations | Investment Securities
Securities Financing Transaction [Line Items]
Available-for-sale, Amortized Cost	67,367	12,542
Available-for-sale, Gross Unrealized Gains	88	0
Available-for-sale, Gross Unrealized Losses	(322)	(318)
Available-for-sale, Estimated Fair Value	67,133	12,224
State and Municipal Obligations | Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	142,168	39,155
Held-to-maturity, Gross Unrealized Gains	610	10
Held-to-maturity, Gross Unrealized Losses	(579)	(856)
Held-to-maturity, Estimated Fair Value	142,199	38,309
Corporate Debt Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	57,000
Held-to-maturity, Estimated Fair Value	53,260
Corporate Debt Securities | Held-to-Maturity Securities | Investment Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	76,033	77,057
Held-to-maturity, Gross Unrealized Gains	361	85
Held-to-maturity, Gross Unrealized Losses	(3,753)	(6,001)
Held-to-maturity, Estimated Fair Value	72,641	71,141
Mortgage-Backed Securities | Held-to-Maturity Securities
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	509,093	462,883
Held-to-maturity, Gross Unrealized Gains	2,496	2,517
Held-to-maturity, Gross Unrealized Losses	(3,653)	(5,345)
Held-to-maturity, Estimated Fair Value	507,936	460,055
Mortgage-Backed Securities | Held-to-Maturity Securities | FHLMC
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	179,890	144,016
Held-to-maturity, Gross Unrealized Gains	447	195
Held-to-maturity, Gross Unrealized Losses	(1,674)	(2,457)
Held-to-maturity, Estimated Fair Value	178,663	141,754
Mortgage-Backed Securities | Held-to-Maturity Securities | FNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	244,279	217,445
Held-to-maturity, Gross Unrealized Gains	1,892	2,175
Held-to-maturity, Gross Unrealized Losses	(1,474)	(2,524)
Held-to-maturity, Estimated Fair Value	244,697	217,096
Mortgage-Backed Securities | Held-to-Maturity Securities | GNMA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	78,441	92,475
Held-to-maturity, Gross Unrealized Gains	96	119
Held-to-maturity, Gross Unrealized Losses	(505)	(364)
Held-to-maturity, Estimated Fair Value	78,032	92,230
Mortgage-Backed Securities | Held-to-Maturity Securities | SBA
Securities Financing Transaction [Line Items]
Held-to-maturity, Amortized Cost	6,483	8,947
Held-to-maturity, Gross Unrealized Gains	61	28
Held-to-maturity, Estimated Fair Value	$ 6,544	$ 8,975