Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 1,850,723	$ 2,078,131
Gross Unrealized Holding Gain, Available-for-sale	1,474	13,282
Gross Unrealized Holding Loss, Available-for-sale	(45,966)	(10,428)
Fair Value, Available-for-sale	$ 1,806,231	$ 2,080,985
Total Percent, Available-for-sale	100.00%	100.00%
Amortized Cost, Held-to-maturity	$ 759,029	$ 829,890
Gross Unrealized Holding Gain, Held-to-maturity	225	4,272
Gross Unrealized Holding Loss, Held-to-maturity	(32,499)	(14,947)
Fair Value, Held-to-maturity	$ 726,755	$ 819,215
Total Percent, Held-to-maturity	100.00%	100.00%
CMO/REMIC - Residential [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 229,832	$ 274,634
Gross Unrealized Holding Gain, Available-for-sale	152	1,277
Gross Unrealized Holding Loss, Available-for-sale	(6,167)	(2,082)
Fair Value, Available-for-sale	$ 223,817	$ 273,829
Total Percent, Available-for-sale	12.39%	13.16%
Amortized Cost, Held-to-maturity	$ 216,980	$ 225,072
Gross Unrealized Holding Gain, Held-to-maturity	0	0
Gross Unrealized Holding Loss, Held-to-maturity	(13,960)	(8,641)
Fair Value, Held-to-maturity	$ 203,020	$ 216,431
Total Percent, Held-to-maturity	28.58%	27.12%
Government Agency/GSE [Member]
Schedule of Investments [Line Items]
Amortized Cost, Held-to-maturity	$ 144,871	$ 159,716
Gross Unrealized Holding Gain, Held-to-maturity	0	854
Gross Unrealized Holding Loss, Held-to-maturity	(5,129)	(2,134)
Fair Value, Held-to-maturity	$ 139,742	$ 158,436
Total Percent, Held-to-maturity	19.09%	19.25%
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 1,570,072	$ 1,747,780
Gross Unrealized Holding Gain, Available-for-sale	1,014	11,231
Gross Unrealized Holding Loss, Available-for-sale	(38,208)	(8,102)
Fair Value, Available-for-sale	$ 1,532,878	$ 1,750,909
Total Percent, Available-for-sale	84.87%	84.14%
Amortized Cost, Held-to-maturity	$ 158,769	$ 176,427
Gross Unrealized Holding Gain, Held-to-maturity	0	667
Gross Unrealized Holding Loss, Held-to-maturity	(5,502)	(382)
Fair Value, Held-to-maturity	$ 153,267	$ 176,712
Total Percent, Held-to-maturity	20.92%	21.26%
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 50,022	$ 54,966
Gross Unrealized Holding Gain, Available-for-sale	308	774
Gross Unrealized Holding Loss, Available-for-sale	(1,591)	(244)
Fair Value, Available-for-sale	$ 48,739	$ 55,496
Total Percent, Available-for-sale	2.70%	2.66%
Amortized Cost, Held-to-maturity	$ 238,409	$ 268,675
Gross Unrealized Holding Gain, Held-to-maturity	225	2,751
Gross Unrealized Holding Loss, Held-to-maturity	(7,908)	(3,790)
Fair Value, Held-to-maturity	$ 230,726	$ 267,636
Total Percent, Held-to-maturity	31.41%	32.37%
Other Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 797	$ 751
Gross Unrealized Holding Gain, Available-for-sale	0	0
Gross Unrealized Holding Loss, Available-for-sale	0	0
Fair Value, Available-for-sale	$ 797	$ 751
Total Percent, Available-for-sale	0.04%	0.04%