Investment Securities - Amortized Cost, Gross Unrealized Gains and Losses and Fair Value of Debt Securities Held to Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021	Jun. 30, 2021
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	$ 3,798,709	$ 3,828,984
Gross Unrealized Gains	38	2,105
Gross Unrealized Losses	(647,667)	(501,619)
Fair Value	3,151,080	3,329,470
Allowance for Credit Losses	1,321	1,646	$ 1,696	$ 1,351	$ 1,165	$ 1,508
Net Carrying Value	3,797,388	3,827,338
U.S. Treasury securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	66,408	66,911
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(753)	(1,022)
Fair Value	65,655	65,889
Allowance for Credit Losses	0	0
Net Carrying Value	66,408	66,911
U.S. government agency securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	306,339	260,392
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(62,428)	(47,680)
Fair Value	243,911	212,712
Allowance for Credit Losses	0	0
Net Carrying Value	306,339	260,392
Obligations of states and state agencies
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	86,968	99,238
Gross Unrealized Gains	5	305
Gross Unrealized Losses	(7,810)	(3,869)
Fair Value	79,163	95,674
Allowance for Credit Losses	415	252
Net Carrying Value	86,553	98,986
Municipal bonds
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	350,673	381,060
Gross Unrealized Gains	5	76
Gross Unrealized Losses	(39,287)	(23,615)
Fair Value	311,391	357,521
Allowance for Credit Losses	57	41
Net Carrying Value	350,616	381,019
Obligations of states and political subdivisions
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	437,641	480,298
Gross Unrealized Gains	10	381
Gross Unrealized Losses	(47,097)	(27,484)
Fair Value	390,554	453,195
Allowance for Credit Losses	472	293
Net Carrying Value	437,169	480,005
Residential mortgage-backed securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	2,875,630	2,909,106
Gross Unrealized Gains	28	1,723
Gross Unrealized Losses	(523,042)	(415,032)
Fair Value	2,352,616	2,495,797
Allowance for Credit Losses	0	0
Net Carrying Value	2,875,630	2,909,106
Trust preferred securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	37,057	37,043
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(6,976)	(5,938)
Fair Value	30,081	31,106
Allowance for Credit Losses	552	888
Net Carrying Value	36,505	36,155
Corporate and other debt securities
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost	75,634	75,234
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(7,371)	(4,463)
Fair Value	68,263	70,771
Allowance for Credit Losses	297	465
Net Carrying Value	$ 75,337	$ 74,769