Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2023	Dec. 31, 2022
Available-for-sale
Amortized Cost	$ 5,743,613	$ 5,930,103
Gross Unrealized Gains	2,028	1,085
Gross Unrealized Losses	(547,328)	(623,881)
Fair Value	5,198,313	5,307,307
Held-to-maturity
Amortized Cost	3,664,393	3,715,052
Gross Unrealized Gains	2,468	1,045
Gross Unrealized Losses	(374,108)	(441,305)
Fair Value	3,292,753	3,274,792
Total debt securities
Amortized Cost	9,408,006	9,645,155
Gross Unrealized Gains	4,496	2,130
Gross Unrealized Losses	(921,436)	(1,065,186)
Fair Value	8,491,066	8,582,099
U.S. government and federal agency
Available-for-sale
Amortized Cost	486,250	487,320
Gross Unrealized Gains	20	23
Gross Unrealized Losses	(36,042)	(42,616)
Fair Value	450,228	444,727
Held-to-maturity
Amortized Cost	848,741	846,046
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(72,382)	(83,796)
Fair Value	776,359	762,250
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	320,614	320,157
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(27,982)	(32,793)
Fair Value	292,632	287,364
State and local governments
Available-for-sale
Amortized Cost	105,171	137,033
Gross Unrealized Gains	696	709
Gross Unrealized Losses	(3,189)	(4,749)
Fair Value	102,678	132,993
Held-to-maturity
Amortized Cost	1,675,146	1,682,640
Gross Unrealized Gains	2,468	1,045
Gross Unrealized Losses	(208,847)	(248,233)
Fair Value	1,468,767	1,435,452
Corporate bonds
Available-for-sale
Amortized Cost	27,049	27,101
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(927)	(992)
Fair Value	26,122	26,109
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	3,572,207	3,706,427
Gross Unrealized Gains	7	6
Gross Unrealized Losses	(392,646)	(439,092)
Fair Value	3,179,568	3,267,341
Held-to-maturity
Amortized Cost	1,140,506	1,186,366
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(92,879)	(109,276)
Fair Value	1,047,627	1,077,090
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,232,322	1,252,065
Gross Unrealized Gains	1,305	347
Gross Unrealized Losses	(86,542)	(103,639)
Fair Value	$ 1,147,085	$ 1,148,773