Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Investment securities available-for-sale:
Total	$ 3,536,760	$ 1,338,260
Gross Unrealized Gain	82,554	34,216
Gross Unrealized Loss	(18,583)	(4,092)
Estimated Fair Value	3,600,731	1,368,384
Investment securities held-to-maturity:
Amortized Cost	27,980	37,838
Gross Unrealized Gain	1,419	922
Gross Unrealized Loss	0	0
Estimated Fair Value	29,399	38,760
Total investment securities
Amortized Cost	3,564,740	1,376,098
Gross Unrealized Gain	83,973	35,138
Gross Unrealized Loss	(18,583)	(4,092)
Estimated Fair Value	3,630,130	1,407,144
U.S. Treasury
Investment securities available-for-sale:
Total	30,161	60,457
Gross Unrealized Gain	2,578	3,137
Gross Unrealized Loss	0	(39)
Estimated Fair Value	32,739	63,555
Agency
Investment securities available-for-sale:
Total	577,627	240,348
Gross Unrealized Gain	26,109	7,686
Gross Unrealized Loss	(691)	(1,676)
Estimated Fair Value	603,045	246,358
Corporate
Investment securities available-for-sale:
Total	397,670	149,150
Gross Unrealized Gain	2,035	2,217
Gross Unrealized Loss	(2,169)	(14)
Estimated Fair Value	397,536	151,353
Municipal bonds
Investment securities available-for-sale:
Total	1,305,389	384,032
Gross Unrealized Gain	23,581	13,450
Gross Unrealized Loss	(15,062)	(184)
Estimated Fair Value	1,313,908	397,298
Collateralized mortgage obligations
Investment securities available-for-sale:
Total	357,615	9,869
Gross Unrealized Gain	1,130	123
Gross Unrealized Loss	(465)	(8)
Estimated Fair Value	358,280	9,984
Mortgage-backed securities
Investment securities available-for-sale:
Total	868,298	494,404
Gross Unrealized Gain	27,121	7,603
Gross Unrealized Loss	(196)	(2,171)
Estimated Fair Value	895,223	499,836
Investment securities held-to-maturity:
Amortized Cost	26,357	36,114
Gross Unrealized Gain	1,419	922
Gross Unrealized Loss	0	0
Estimated Fair Value	27,776	37,036
Other
Investment securities held-to-maturity:
Amortized Cost	1,623	1,724
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 1,623	$ 1,724

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