Investment Securities - Amortized Cost and Estimated Fair Value of Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Investment securities available-for-sale:
Amortized Cost	$ 3,846,733	$ 1,338,260
Gross Unrealized Gain	90,004	34,216
Gross Unrealized Gain	(5,622)	(4,092)
Estimated Fair Value	3,931,115	1,368,384
Investment securities held-to-maturity:
Amortized Cost	23,732	37,838
Gross Unrealized Gain	1,281	922
Gross Unrealized Loss	0	0
Estimated Fair Value	25,013	38,760
Total investment securities
Amortized Cost	3,870,465	1,376,098
Gross Unrealized Gain	91,285	35,138
Gross Unrealized Loss	(5,622)	(4,092)
Estimated Fair Value	3,956,128	1,407,144
U.S. Treasury
Investment securities available-for-sale:
Amortized Cost	30,153	60,457
Gross Unrealized Gain	2,380	3,137
Gross Unrealized Gain	0	(39)
Estimated Fair Value	32,533	63,555
Agency
Investment securities available-for-sale:
Amortized Cost	666,702	240,348
Gross Unrealized Gain	24,292	7,686
Gross Unrealized Gain	(608)	(1,676)
Estimated Fair Value	690,386	246,358
Corporate
Investment securities available-for-sale:
Amortized Cost	412,223	149,150
Gross Unrealized Gain	3,591	2,217
Gross Unrealized Gain	(506)	(14)
Estimated Fair Value	415,308	151,353
Municipal bonds
Investment securities available-for-sale:
Amortized Cost	1,412,012	384,032
Gross Unrealized Gain	37,260	13,450
Gross Unrealized Gain	(3,253)	(184)
Estimated Fair Value	1,446,019	397,298
Collateralized mortgage obligation
Investment securities available-for-sale:
Amortized Cost	513,259	9,869
Gross Unrealized Gain	819	123
Gross Unrealized Gain	(712)	(8)
Estimated Fair Value	513,366	9,984
Mortgage-backed securities
Investment securities available-for-sale:
Amortized Cost	812,384	494,404
Gross Unrealized Gain	21,662	7,603
Gross Unrealized Gain	(543)	(2,171)
Estimated Fair Value	833,503	499,836
Investment securities held-to-maturity:
Amortized Cost	22,124	36,114
Gross Unrealized Gain	1,281	922
Gross Unrealized Loss	0	0
Estimated Fair Value	23,405	37,036
Other
Investment securities held-to-maturity:
Amortized Cost	1,608	1,724
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 1,608	$ 1,724

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