Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Investment securities available-for-sale:
Total	$ 2,254,484	$ 1,338,260
Gross Unrealized Gain	84,081	34,216
Gross Unrealized Loss	(2,499)	(4,092)
Estimated Fair Value	2,336,066	1,368,384
Investment securities held-to-maturity:
Amortized Cost	32,557	37,838
Gross Unrealized Gain	1,622	922
Gross Unrealized Loss	0	0
Estimated Fair Value	34,179	38,760
Total investment securities
Amortized Cost	2,287,041	1,376,098
Gross Unrealized Gain	85,703	35,138
Gross Unrealized Loss	(2,499)	(4,092)
Estimated Fair Value	2,370,245	1,407,144
U.S. Treasury
Investment securities available-for-sale:
Total	30,170	60,457
Gross Unrealized Gain	2,700	3,137
Gross Unrealized Loss	0	(39)
Estimated Fair Value	32,870	63,555
Agency
Investment securities available-for-sale:
Total	315,927	240,348
Gross Unrealized Gain	24,419	7,686
Gross Unrealized Loss	(356)	(1,676)
Estimated Fair Value	339,990	246,358
Corporate
Investment securities available-for-sale:
Total	274,076	149,150
Gross Unrealized Gain	1,698	2,217
Gross Unrealized Loss	(1,775)	(14)
Estimated Fair Value	273,999	151,353
Municipal bonds
Investment securities available-for-sale:
Total	439,772	384,032
Gross Unrealized Gain	21,629	13,450
Gross Unrealized Loss	(66)	(184)
Estimated Fair Value	461,335	397,298
Collateralized mortgage obligations
Investment securities available-for-sale:
Total	380,527	9,869
Gross Unrealized Gain	4,260	123
Gross Unrealized Loss	(48)	(8)
Estimated Fair Value	384,739	9,984
Mortgage-backed securities
Investment securities available-for-sale:
Total	814,012	494,404
Gross Unrealized Gain	29,375	7,603
Gross Unrealized Loss	(254)	(2,171)
Estimated Fair Value	843,133	499,836
Investment securities held-to-maturity:
Amortized Cost	30,892	36,114
Gross Unrealized Gain	1,622	922
Gross Unrealized Loss	0	0
Estimated Fair Value	32,514	37,036
Other
Investment securities held-to-maturity:
Amortized Cost	1,665	1,724
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 1,665	$ 1,724

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Available-for-sale Securities And Held-To-Maturity Securities, Accumulated Gross Unrealized Gain, before Tax

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