Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2025	Dec. 31, 2024	Sep. 30, 2024	Jun. 30, 2024	Dec. 31, 2023	Jun. 30, 2023
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,541,456	$ 3,532,220
Gross Unrealized Gains	9,846	2,107
Gross Unrealized Losses	(400,779)	(506,043)
Fair Value	3,150,523	3,028,284
Allowance for Credit Losses	637	647	$ 1,076	$ 637	$ 1,205	$ 1,090
Net Carrying Value	3,540,819	3,531,573
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	292,897	301,315
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(39,617)	(49,013)
Fair Value	253,280	252,302
Allowance for Credit Losses	0	0
Net Carrying Value	292,897	301,315
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	62,184	68,025
Gross Unrealized Gains	384	0
Gross Unrealized Losses	(3,512)	(5,335)
Fair Value	59,056	62,690
Allowance for Credit Losses	1	2
Net Carrying Value	62,183	68,023
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	287,789	304,464
Gross Unrealized Gains	22	9
Gross Unrealized Losses	(20,734)	(20,802)
Fair Value	267,077	283,671
Allowance for Credit Losses	69	48
Net Carrying Value	287,720	304,416
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	349,973	372,489
Gross Unrealized Gains	406	9
Gross Unrealized Losses	(24,246)	(26,137)
Fair Value	326,133	346,361
Allowance for Credit Losses	70	50
Net Carrying Value	349,903	372,439
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,777,381	2,710,642
Gross Unrealized Gains	9,440	2,088
Gross Unrealized Losses	(329,593)	(420,582)
Fair Value	2,457,228	2,292,148
Allowance for Credit Losses	0	0
Net Carrying Value	2,777,381	2,710,642
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	36,097	36,081
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(5,511)	(6,936)
Fair Value	30,586	29,145
Allowance for Credit Losses	411	414
Net Carrying Value	35,686	35,667
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	85,108	86,213
Gross Unrealized Gains	0	10
Gross Unrealized Losses	(1,812)	(3,356)
Fair Value	83,296	82,867
Allowance for Credit Losses	156	183
Net Carrying Value	$ 84,952	86,030
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost		25,480
Gross Unrealized Gains		0
Gross Unrealized Losses		(19)
Fair Value		25,461
Allowance for Credit Losses		0
Net Carrying Value		$ 25,480