Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Jun. 30, 2021	Dec. 31, 2020
Investment securities available-for-sale:
Total	$ 4,450,349	$ 3,846,733
Gross Unrealized Gain	62,099	90,004
Gross Unrealized Loss	(25,001)	(5,622)
Estimated Fair Value	4,487,447	3,931,115
Investment securities held-to-maturity:
Amortized Cost	18,933	23,732
Gross Unrealized Gain	887	1,281
Gross Unrealized Loss	0	0
Estimated Fair Value	19,820	25,013
Total investment securities
Amortized Cost	4,469,282	3,870,465
Gross Unrealized Gain	62,986	91,285
Gross Unrealized Loss	(25,001)	(5,622)
Estimated Fair Value	4,507,267	3,956,128
U.S. Treasury
Investment securities available-for-sale:
Total	109,847	30,153
Gross Unrealized Gain	2,008	2,380
Gross Unrealized Loss	(47)	0
Estimated Fair Value	111,808	32,533
Agency
Investment securities available-for-sale:
Total	554,342	666,702
Gross Unrealized Gain	6,335	24,292
Gross Unrealized Loss	(5,510)	(608)
Estimated Fair Value	555,167	690,386
Corporate
Investment securities available-for-sale:
Total	355,101	412,223
Gross Unrealized Gain	5,957	3,591
Gross Unrealized Loss	(2,676)	(506)
Estimated Fair Value	358,382	415,308
Municipal bonds
Investment securities available-for-sale:
Total	1,348,346	1,412,012
Gross Unrealized Gain	36,296	37,260
Gross Unrealized Loss	(5,169)	(3,253)
Estimated Fair Value	1,379,473	1,446,019
Collateralized mortgage obligations
Investment securities available-for-sale:
Total	617,596	513,259
Gross Unrealized Gain	287	819
Gross Unrealized Loss	(3,145)	(712)
Estimated Fair Value	614,738	513,366
Mortgage-backed securities
Investment securities available-for-sale:
Total	1,465,117	812,384
Gross Unrealized Gain	11,216	21,662
Gross Unrealized Loss	(8,454)	(543)
Estimated Fair Value	1,467,879	833,503
Investment securities held-to-maturity:
Amortized Cost	17,378	22,124
Gross Unrealized Gain	887	1,281
Gross Unrealized Loss	0	0
Estimated Fair Value	18,265	23,405
Other
Investment securities held-to-maturity:
Amortized Cost	1,555	1,608
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 1,555	$ 1,608

Available-for-sale and Held-to-maturity Securities [Abstract]

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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