Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 1,757,666	$ 2,078,131
Gross Unrealized Holding Gain, Available-for-sale	2,033	13,282
Gross Unrealized Holding Loss, Available-for-sale	(25,614)	(10,428)
Fair Value, Available-for-sale	$ 1,734,085	$ 2,080,985
Total Percent, Available-for-sale	100.00%	100.00%
Amortized Cost, Held-to-maturity	$ 744,440	$ 829,890
Gross Unrealized Holding Gain, Held-to-maturity	1,128	4,272
Gross Unrealized Holding Loss, Held-to-maturity	(24,031)	(14,947)
Fair Value, Held-to-maturity	$ 721,537	$ 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	$ 217,223	$ 274,634
Gross Unrealized Holding Gain, Available-for-sale	353	1,277
Gross Unrealized Holding Loss, Available-for-sale	(3,525)	(2,082)
Fair Value, Available-for-sale	$ 214,051	$ 273,829
Total Percent, Available-for-sale	12.34%	13.16%
Amortized Cost, Held-to-maturity	$ 215,336	$ 225,072
Gross Unrealized Holding Gain, Held-to-maturity	0	0
Gross Unrealized Holding Loss, Held-to-maturity	(12,081)	(8,641)
Fair Value, Held-to-maturity	$ 203,255	$ 216,431
Total Percent, Held-to-maturity	28.93%	27.12%
Government Agency/GSE [Member]
Schedule of Investments [Line Items]
Amortized Cost, Held-to-maturity	$ 138,274	$ 159,716
Gross Unrealized Holding Gain, Held-to-maturity	572	854
Gross Unrealized Holding Loss, Held-to-maturity	(2,622)	(2,134)
Fair Value, Held-to-maturity	$ 136,224	$ 158,436
Total Percent, Held-to-maturity	18.57%	19.25%
Residential Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 1,494,106	$ 1,747,780
Gross Unrealized Holding Gain, Available-for-sale	1,348	11,231
Gross Unrealized Holding Loss, Available-for-sale	(20,946)	(8,102)
Fair Value, Available-for-sale	$ 1,474,508	$ 1,750,909
Total Percent, Available-for-sale	85.03%	84.14%
Amortized Cost, Held-to-maturity	$ 153,874	$ 176,427
Gross Unrealized Holding Gain, Held-to-maturity	0	667
Gross Unrealized Holding Loss, Held-to-maturity	(3,140)	(382)
Fair Value, Held-to-maturity	$ 150,734	$ 176,712
Total Percent, Held-to-maturity	20.67%	21.26%
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 45,621	$ 54,966
Gross Unrealized Holding Gain, Available-for-sale	332	774
Gross Unrealized Holding Loss, Available-for-sale	(1,143)	(244)
Fair Value, Available-for-sale	$ 44,810	$ 55,496
Total Percent, Available-for-sale	2.59%	2.66%
Amortized Cost, Held-to-maturity	$ 236,956	$ 268,675
Gross Unrealized Holding Gain, Held-to-maturity	556	2,751
Gross Unrealized Holding Loss, Held-to-maturity	(6,188)	(3,790)
Fair Value, Held-to-maturity	$ 231,324	$ 267,636
Total Percent, Held-to-maturity	31.83%	32.37%
Other Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 716	$ 751
Gross Unrealized Holding Gain, Available-for-sale	0	0
Gross Unrealized Holding Loss, Available-for-sale	0	0
Fair Value, Available-for-sale	$ 716	$ 751
Total Percent, Available-for-sale	0.04%	0.04%