Investment Securities - Summary of Amortized Cost and Estimated Fair Value of Investment Securities (Detail) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 2,797,990	$ 2,344,174
Gross Unrealized Holding Gain, Available-for-sale	43,588	55,099
Gross Unrealized Holding Loss, Available-for-sale	(29,230)	(350)
Fair Value, Available-for-sale	$ 2,812,348	$ 2,398,923
Total Percent, Available-for-sale	100.00%	100.00%
Amortized Cost, Held-to-maturity	$ 1,086,984	$ 578,626
Gross Unrealized Holding Gain, Held-to-maturity	17,135	25,703
Gross Unrealized Holding Loss, Held-to-maturity	(18,638)	(106)
Fair Value, Held-to-maturity	$ 1,085,481	$ 604,223
Total Percent, Held-to-maturity	100.00%	100.00%
CMO/REMIC [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 592,915	$ 457,548
Gross Unrealized Holding Gain, Available-for-sale	4,072	5,515
Gross Unrealized Holding Loss, Available-for-sale	(6,821)	(249)
Fair Value, Available-for-sale	$ 590,166	$ 462,814
Total Percent, Available-for-sale	20.99%	19.29%
Amortized Cost, Held-to-maturity	$ 133,556	$ 145,309
Gross Unrealized Holding Gain, Held-to-maturity	2,832	5,202
Gross Unrealized Holding Loss, Held-to-maturity		0
Fair Value, Held-to-maturity	$ 136,388	$ 150,511
Total Percent, Held-to-maturity	12.29%	25.11%
Government Agency/GSE [Member]
Schedule of Investments [Line Items]
Amortized Cost, Held-to-maturity	$ 601,142	$ 98,663
Gross Unrealized Holding Gain, Held-to-maturity	3,860	5,877
Gross Unrealized Holding Loss, Held-to-maturity		0
Fair Value, Held-to-maturity	$ 588,713	$ 104,540
Total Percent, Held-to-maturity	55.30%	17.05%
Mortgage-backed Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 2,175,414	$ 1,857,030
Gross Unrealized Holding Gain, Available-for-sale	38,333	48,006
Gross Unrealized Holding Loss, Available-for-sale	(22,409)	(101)
Fair Value, Available-for-sale	$ 2,191,338	$ 1,904,935
Total Percent, Available-for-sale	77.92%	79.41%
Amortized Cost, Held-to-maturity	$ 135,137	$ 146,382
Gross Unrealized Holding Gain, Held-to-maturity	5,348	7,644
Gross Unrealized Holding Loss, Held-to-maturity		(32)
Fair Value, Held-to-maturity	$ 140,230	$ 153,994
Total Percent, Held-to-maturity	12.43%	25.30%
Municipal Bonds [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 28,703	$ 28,707
Gross Unrealized Holding Gain, Available-for-sale	1,183	1,578
Gross Unrealized Holding Loss, Available-for-sale		0
Fair Value, Available-for-sale	$ 29,886	$ 30,285
Total Percent, Available-for-sale	1.06%	1.26%
Amortized Cost, Held-to-maturity	$ 217,149	$ 188,272
Gross Unrealized Holding Gain, Held-to-maturity	5,095	6,980
Gross Unrealized Holding Loss, Held-to-maturity	(2,094)	(74)
Fair Value, Held-to-maturity	$ 220,150	$ 195,178
Total Percent, Held-to-maturity	19.98%	32.54%
Other Securities [Member]
Schedule of Investments [Line Items]
Amortized Cost, Available-for-sale	$ 958	$ 889
Gross Unrealized Holding Gain, Available-for-sale
Gross Unrealized Holding Loss, Available-for-sale
Fair Value, Available-for-sale	$ 958	$ 889
Total Percent, Available-for-sale	0.03%	0.04%