Debt Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Available-for-sale
Amortized Cost	$ 4,785,172	$ 5,283,253
Gross Unrealized Gains	1,039	841
Gross Unrealized Losses	(349,633)	(498,375)
Fair Value	4,436,578	4,785,719
Held-to-maturity
Amortized Cost	3,348,698	3,502,411
Gross Unrealized Gains	1,874	2,843
Gross Unrealized Losses	(270,023)	(325,060)
Fair Value	3,080,549	3,180,194
Total debt securities
Amortized Cost	8,133,870	8,785,664
Gross Unrealized Gains	2,913	3,684
Gross Unrealized Losses	(619,656)	(823,435)
Fair Value	7,517,127	7,965,913
U.S. government and federal agency
Available-for-sale
Amortized Cost	484,314	485,005
Gross Unrealized Gains	17	11
Gross Unrealized Losses	(16,725)	(29,669)
Fair Value	467,606	455,347
Held-to-maturity
Amortized Cost	857,858	853,273
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(43,408)	(65,472)
Fair Value	814,450	787,801
U.S. government sponsored enterprises
Available-for-sale
Amortized Cost	323,387	321,993
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(13,318)	(22,774)
Fair Value	310,069	299,219
State and local governments
Available-for-sale
Amortized Cost	74,691	101,903
Gross Unrealized Gains	340	302
Gross Unrealized Losses	(2,661)	(3,273)
Fair Value	72,370	98,932
Held-to-maturity
Amortized Cost	1,630,264	1,650,000
Gross Unrealized Gains	1,874	2,843
Gross Unrealized Losses	(172,903)	(181,192)
Fair Value	1,459,235	1,471,651
Corporate bonds
Available-for-sale
Amortized Cost	14,838	27,007
Gross Unrealized Gains	0	2
Gross Unrealized Losses	(296)	(756)
Fair Value	14,542	26,253
Residential mortgage-backed securities
Available-for-sale
Amortized Cost	2,780,411	3,166,589
Gross Unrealized Gains	18	7
Gross Unrealized Losses	(256,225)	(355,333)
Fair Value	2,524,204	2,811,263
Held-to-maturity
Amortized Cost	860,576	999,138
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(53,712)	(78,396)
Fair Value	806,864	920,742
Commercial mortgage-backed securities
Available-for-sale
Amortized Cost	1,107,531	1,180,756
Gross Unrealized Gains	664	519
Gross Unrealized Losses	(60,408)	(86,570)
Fair Value	$ 1,047,787	$ 1,094,705