Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Available-for-sale
Amortized Cost	$ 5,410,487	$ 5,930,103
Gross Unrealized Gains	240	1,085
Gross Unrealized Losses	(668,989)	(623,881)
Fair Value	4,741,738	5,307,307
Held-to-maturity
Amortized Cost	3,553,805	3,715,052
Gross Unrealized Gains	63	1,045
Gross Unrealized Losses	(515,401)	(441,305)
Fair Value	3,038,467	3,274,792
Total debt securities
Amortized Cost	8,964,292	9,645,155
Gross Unrealized Gains	303	2,130
Gross Unrealized Losses	(1,184,390)	(1,065,186)
Fair Value	7,780,205	8,582,099
U.S. government and federal agency
Available-for-sale
Amortized Cost	485,496	487,320
Gross Unrealized Gains	13	23
Gross Unrealized Losses	(41,364)	(42,616)
Fair Value	444,145	444,727
Held-to-maturity
Amortized Cost	851,751	846,046
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(93,899)	(83,796)
Fair Value	757,852	762,250
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	321,532	320,157
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(31,701)	(32,793)
Fair Value	289,831	287,364
State and local governments
Available-for-sale
Amortized Cost	102,702	137,033
Gross Unrealized Gains	96	709
Gross Unrealized Losses	(6,085)	(4,749)
Fair Value	96,713	132,993
Held-to-maturity
Amortized Cost	1,657,628	1,682,640
Gross Unrealized Gains	63	1,045
Gross Unrealized Losses	(311,437)	(248,233)
Fair Value	1,346,254	1,435,452
Corporate bonds
Available-for-sale
Amortized Cost	27,021	27,101
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1,117)	(992)
Fair Value	25,904	26,109
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	3,281,286	3,706,427
Gross Unrealized Gains	2	6
Gross Unrealized Losses	(470,394)	(439,092)
Fair Value	2,810,894	3,267,341
Held-to-maturity
Amortized Cost	1,044,426	1,186,366
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(110,065)	(109,276)
Fair Value	934,361	1,077,090
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,192,450	1,252,065
Gross Unrealized Gains	129	347
Gross Unrealized Losses	(118,328)	(103,639)
Fair Value	$ 1,074,251	$ 1,148,773