Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2024	Dec. 31, 2023
Available-for-sale
Amortized Cost	$ 5,124,600	$ 5,283,253
Gross Unrealized Gains	376	841
Gross Unrealized Losses	(495,903)	(498,375)
Fair Value	4,629,073	4,785,719
Held-to-maturity
Amortized Cost	3,451,583	3,502,411
Gross Unrealized Gains	1,344	2,843
Gross Unrealized Losses	(361,733)	(325,060)
Fair Value	3,091,194	3,180,194
Total debt securities
Amortized Cost	8,576,183	8,785,664
Gross Unrealized Gains	1,720	3,684
Gross Unrealized Losses	(857,636)	(823,435)
Fair Value	7,720,267	7,965,913
U.S. government and federal agency
Available-for-sale
Amortized Cost	484,765	485,005
Gross Unrealized Gains	9	11
Gross Unrealized Losses	(29,957)	(29,669)
Fair Value	454,817	455,347
Held-to-maturity
Amortized Cost	854,791	853,273
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(71,911)	(65,472)
Fair Value	782,880	787,801
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	329,392	321,993
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(22,766)	(22,774)
Fair Value	306,626	299,219
State and local governments
Available-for-sale
Amortized Cost	101,090	101,903
Gross Unrealized Gains	74	302
Gross Unrealized Losses	(3,782)	(3,273)
Fair Value	97,382	98,932
Held-to-maturity
Amortized Cost	1,641,551	1,650,000
Gross Unrealized Gains	1,344	2,843
Gross Unrealized Losses	(205,613)	(181,192)
Fair Value	1,437,282	1,471,651
Corporate bonds
Available-for-sale
Amortized Cost	14,881	27,007
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(675)	(756)
Fair Value	14,206	26,253
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	3,028,348	3,166,589
Gross Unrealized Gains	3	7
Gross Unrealized Losses	(350,700)	(355,333)
Fair Value	2,677,651	2,811,263
Held-to-maturity
Amortized Cost	955,241	999,138
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(84,209)	(78,396)
Fair Value	871,032	920,742
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,166,124	1,180,756
Gross Unrealized Gains	290	519
Gross Unrealized Losses	(88,023)	(86,570)
Fair Value	$ 1,078,391	$ 1,094,705