Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Investment securities available-for-sale:
Total	$ 1,275,549	$ 1,338,260
Gross Unrealized Gain	63,795	34,216
Gross Unrealized Loss	(1,583)	(4,092)
Estimated Fair Value	1,337,761	1,368,384
Investment securities held-to-maturity:
Amortized Cost	34,553	37,838
Gross Unrealized Gain	1,685	922
Gross Unrealized Loss	0	0
Estimated Fair Value	36,238	38,760
Total investment securities
Amortized Cost	1,310,102	1,376,098
Gross Unrealized Gain	65,480	35,138
Gross Unrealized Loss	(1,583)	(4,092)
Estimated Fair Value	1,373,999	1,407,144
U.S. Treasury
Investment securities available-for-sale:
Total	30,179	60,457
Gross Unrealized Gain	2,654	3,137
Gross Unrealized Loss	0	(39)
Estimated Fair Value	32,833	63,555
Agency
Investment securities available-for-sale:
Total	228,715	240,348
Gross Unrealized Gain	19,350	7,686
Gross Unrealized Loss	(299)	(1,676)
Estimated Fair Value	247,766	246,358
Corporate
Investment securities available-for-sale:
Total	174,161	149,150
Gross Unrealized Gain	2,658	2,217
Gross Unrealized Loss	(786)	(14)
Estimated Fair Value	176,033	151,353
Municipal bonds
Investment securities available-for-sale:
Total	391,992	384,032
Gross Unrealized Gain	15,602	13,450
Gross Unrealized Loss	(497)	(184)
Estimated Fair Value	407,097	397,298
Collateralized mortgage obligations
Investment securities available-for-sale:
Total	9,346	9,869
Gross Unrealized Gain	311	123
Gross Unrealized Loss	(1)	(8)
Estimated Fair Value	9,656	9,984
Mortgage-backed securities
Investment securities available-for-sale:
Total	441,156	494,404
Gross Unrealized Gain	23,220	7,603
Gross Unrealized Loss	0	(2,171)
Estimated Fair Value	464,376	499,836
Investment securities held-to-maturity:
Amortized Cost	32,865	36,114
Gross Unrealized Gain	1,685	922
Gross Unrealized Loss	0	0
Estimated Fair Value	34,550	37,036
Other
Investment securities held-to-maturity:
Amortized Cost	1,688	1,724
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 1,688	$ 1,724

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