Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Jun. 30, 2023	Dec. 31, 2022	Sep. 30, 2022	Jun. 30, 2022	Dec. 31, 2021
Held-to-maturity
Amortized Cost	$ 3,745,506		$ 3,761,094
Allowance for credit losses on HTM	(3,214)	$ (3,214)	(1,388)	$ (1,384)	$ (1,381)	$ (1,279)
Net Carrying Amount	3,742,292		3,759,706
Gross Unrealized Gains	26		283
Gross Unrealized (Losses)	(894,107)		(696,756)
Estimated Fair Value	2,848,211		3,063,233
Available-for-sale
Amortized Cost	3,890,910		4,331,413
Allowance for Credit Losses	(1,196)	(2,396)	0
Gross Unrealized Gains	156		373
Gross Unrealized (Losses)	(531,449)		(478,932)
Estimated Fair Value	3,358,421		3,852,854
U.S. Government agencies
Held-to-maturity
Amortized Cost	452,428		448,012
Allowance for credit losses on HTM	0		0
Net Carrying Amount	452,428		448,012
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(117,530)		(102,558)
Estimated Fair Value	334,898		345,454
Available-for-sale
Amortized Cost	179,638		191,498
Allowance for Credit Losses	0		0
Gross Unrealized Gains	45		103
Gross Unrealized (Losses)	(6,924)		(7,322)
Estimated Fair Value	172,759		184,279
U.S. Treasury
Available-for-sale
Amortized Cost	2,278		2,257
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(54)		(60)
Estimated Fair Value	2,224		2,197
Mortgage-backed securities
Held-to-maturity
Amortized Cost	1,178,324		1,190,781
Allowance for credit losses on HTM	0		0
Net Carrying Amount	1,178,324		1,190,781
Gross Unrealized Gains	0		227
Gross Unrealized (Losses)	(172,976)		(118,960)
Estimated Fair Value	1,005,348		1,072,048
Available-for-sale
Amortized Cost	2,444,803		2,809,319
Allowance for Credit Losses	0		0
Gross Unrealized Gains	0		20
Gross Unrealized (Losses)	(287,711)		(266,437)
Estimated Fair Value	2,157,092		2,542,902
State and political subdivisions
Held-to-maturity
Amortized Cost	1,859,172		1,861,102
Allowance for credit losses on HTM	(1,520)	(934)	(110)	(107)	(103)	(1,197)
Net Carrying Amount	1,857,652		1,860,992
Gross Unrealized Gains	26		56
Gross Unrealized (Losses)	(564,366)		(446,198)
Estimated Fair Value	1,293,312		1,414,850
Available-for-sale
Amortized Cost	999,373		1,056,124
Allowance for Credit Losses	0	0	0
Gross Unrealized Gains	111		250
Gross Unrealized (Losses)	(209,140)		(185,300)
Estimated Fair Value	790,344		871,074
Other securities
Held-to-maturity
Amortized Cost	255,582		261,199
Allowance for credit losses on HTM	(1,694)	(2,280)	(1,278)	$ (1,277)	$ (1,278)	$ (82)
Net Carrying Amount	253,888		259,921
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(39,235)		(29,040)
Estimated Fair Value	214,653		230,881
Available-for-sale
Amortized Cost	264,818		272,215
Allowance for Credit Losses	(1,196)	$ (2,396)	0
Gross Unrealized Gains	0		0
Gross Unrealized (Losses)	(27,620)		(19,813)
Estimated Fair Value	$ 236,002		$ 252,402