Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Jun. 30, 2025	Dec. 31, 2024	Jun. 30, 2024	Mar. 31, 2024	Dec. 31, 2023	Mar. 31, 2023
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,531,561	$ 3,532,220
Gross Unrealized Gains	6,751	2,107
Gross Unrealized Losses	(449,911)	(506,043)
Fair Value	3,088,401	3,028,284
Allowance for Credit Losses	637	647	$ 1,090	$ 633	$ 1,205	$ 1,131
Net Carrying Value	3,530,924	3,531,573
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	25,095	25,480
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(16)	(19)
Fair Value	25,079	25,461
Allowance for Credit Losses	0	0
Net Carrying Value	25,095	25,480
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	297,057	301,315
Gross Unrealized Gains	6	0
Gross Unrealized Losses	(43,324)	(49,013)
Fair Value	253,739	252,302
Allowance for Credit Losses	0	0
Net Carrying Value	297,057	301,315
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	64,729	68,025
Gross Unrealized Gains	167	0
Gross Unrealized Losses	(4,457)	(5,335)
Fair Value	60,439	62,690
Allowance for Credit Losses	1	2
Net Carrying Value	64,728	68,023
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	293,648	304,464
Gross Unrealized Gains	13	9
Gross Unrealized Losses	(25,606)	(20,802)
Fair Value	268,055	283,671
Allowance for Credit Losses	74	48
Net Carrying Value	293,574	304,416
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	358,377	372,489
Gross Unrealized Gains	180	9
Gross Unrealized Losses	(30,063)	(26,137)
Fair Value	328,494	346,361
Allowance for Credit Losses	75	50
Net Carrying Value	358,302	372,439
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,731,797	2,710,642
Gross Unrealized Gains	6,564	2,088
Gross Unrealized Losses	(367,800)	(420,582)
Fair Value	2,370,561	2,292,148
Allowance for Credit Losses	0	0
Net Carrying Value	2,731,797	2,710,642
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	36,092	36,081
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(6,364)	(6,936)
Fair Value	29,728	29,145
Allowance for Credit Losses	408	414
Net Carrying Value	35,684	35,667
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	83,143	86,213
Gross Unrealized Gains	1	10
Gross Unrealized Losses	(2,344)	(3,356)
Fair Value	80,800	82,867
Allowance for Credit Losses	154	183
Net Carrying Value	$ 82,989	$ 86,030