Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Available-for-sale
Amortized Cost	$ 6,411,714	$ 9,175,423
Gross Unrealized Gains	2,256	68,117
Gross Unrealized Losses	(658,894)	(72,691)
Fair Value	5,755,076	9,170,849
Held-to-maturity
Amortized Cost	3,756,634	1,199,164
Gross Unrealized Gains	169	22,878
Gross Unrealized Losses	(498,862)	(1,159)
Fair Value	3,257,941	1,220,883
Total debt securities
Amortized Cost	10,168,348	10,374,587
Gross Unrealized Gains	2,425	90,995
Gross Unrealized Losses	(1,157,756)	(73,850)
Fair Value	9,013,017	10,391,732
U.S. government and federal agency
Available-for-sale
Amortized Cost	488,244	1,356,171
Gross Unrealized Gains	37	174
Gross Unrealized Losses	(44,815)	(9,596)
Fair Value	443,466	1,346,749
Held-to-maturity
Amortized Cost	845,116
Gross Unrealized Gains	0
Gross Unrealized Losses	(86,630)
Fair Value	758,486
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	319,962	241,687
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(31,430)	(996)
Fair Value	288,532	240,693
State and local governments
Available-for-sale
Amortized Cost	421,886	461,414
Gross Unrealized Gains	1,243	27,567
Gross Unrealized Losses	(11,274)	(123)
Fair Value	411,855	488,858
Held-to-maturity
Amortized Cost	1,675,529	1,199,164
Gross Unrealized Gains	169	22,878
Gross Unrealized Losses	(302,050)	(1,159)
Fair Value	1,373,648	1,220,883
Corporate bonds
Available-for-sale
Amortized Cost	84,451	175,697
Gross Unrealized Gains	421	5,072
Gross Unrealized Losses	(1,141)	(17)
Fair Value	83,731	180,752
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	3,855,896	5,744,505
Gross Unrealized Gains	12	9,420
Gross Unrealized Losses	(468,530)	(54,266)
Fair Value	3,387,378	5,699,659
Held-to-maturity
Amortized Cost	1,235,989
Gross Unrealized Gains	0
Gross Unrealized Losses	(110,182)
Fair Value	1,125,807
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,241,275	1,195,949
Gross Unrealized Gains	543	25,882
Gross Unrealized Losses	(101,704)	(7,693)
Fair Value	$ 1,140,114	$ 1,214,138