Investment Securities - Amortized Cost and Estimated Fair Value (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Investment securities available-for-sale:
Total	$ 3,878,665	$ 3,846,733
Gross Unrealized Gain	49,030	90,004
Gross Unrealized Loss	(70,358)	(5,622)
Estimated Fair Value	3,857,337	3,931,115
Investment securities held-to-maturity:
Amortized Cost	21,931	23,732
Gross Unrealized Gain	1,073	1,281
Gross Unrealized Loss	0	0
Estimated Fair Value	23,004	25,013
Total investment securities
Amortized Cost	3,900,596	3,870,465
Gross Unrealized Gain	50,103	91,285
Gross Unrealized Loss	(70,358)	(5,622)
Estimated Fair Value	3,880,341	3,956,128
U.S. Treasury
Investment securities available-for-sale:
Total	30,144	30,153
Gross Unrealized Gain	1,919	2,380
Gross Unrealized Loss	0	0
Estimated Fair Value	32,063	32,533
Agency
Investment securities available-for-sale:
Total	660,582	666,702
Gross Unrealized Gain	12,956	24,292
Gross Unrealized Loss	(10,379)	(608)
Estimated Fair Value	663,159	690,386
Corporate
Investment securities available-for-sale:
Total	360,617	412,223
Gross Unrealized Gain	2,128	3,591
Gross Unrealized Loss	(4,911)	(506)
Estimated Fair Value	357,834	415,308
Municipal bonds
Investment securities available-for-sale:
Total	1,432,169	1,412,012
Gross Unrealized Gain	22,287	37,260
Gross Unrealized Loss	(32,394)	(3,253)
Estimated Fair Value	1,422,062	1,446,019
Collateralized mortgage obligations
Investment securities available-for-sale:
Total	514,250	513,259
Gross Unrealized Gain	431	819
Gross Unrealized Loss	(5,710)	(712)
Estimated Fair Value	508,971	513,366
Mortgage-backed securities
Investment securities available-for-sale:
Total	880,903	812,384
Gross Unrealized Gain	9,309	21,662
Gross Unrealized Loss	(16,964)	(543)
Estimated Fair Value	873,248	833,503
Investment securities held-to-maturity:
Amortized Cost	20,353	22,124
Gross Unrealized Gain	1,073	1,281
Gross Unrealized Loss	0	0
Estimated Fair Value	21,426	23,405
Other
Investment securities held-to-maturity:
Amortized Cost	1,578	1,608
Gross Unrealized Gain	0	0
Gross Unrealized Loss	0	0
Estimated Fair Value	$ 1,578	$ 1,608