Investment Securities - Amortized Cost and Fair Value of Available-for-Sale Investment Securities Portfolio (Details) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Available-for-Sale
Total	$ 7,719,944	$ 7,772,603
Unrealized Gains	234	3,028
Unrealized Losses	(1,143,301)	(847,474)
Basis adjustments	(162,116)	(154,445)
Fair Value	6,414,761	6,773,712
Held-to-Maturity
Allowance for securities held-to-maturity	(150)	(150)
Total held-to-maturity securities	3,027,914	3,089,147
Unrealized Gains	0	221
Unrealized Losses	(634,293)	(445,686)
Basis adjustments	0	0
Fair Value	2,393,621	2,643,682
U.S. Treasury
Available-for-Sale
Total	616,608	253,148
Unrealized Gains	98	5
Unrealized Losses	(10,258)	(5,189)
Basis adjustments	(58,322)	(47,037)
Fair Value	548,126	200,927
U.S. government-sponsored entities and agencies
Available-for-Sale
Total	1,475,307	1,451,736
Unrealized Gains	0	0
Unrealized Losses	(229,113)	(169,248)
Basis adjustments	(103,294)	(107,408)
Fair Value	1,142,900	1,175,080
Held-to-Maturity
Amortized Cost	824,223	819,168
Unrealized Gains	0	0
Unrealized Losses	(207,213)	(162,810)
Basis adjustments	0	0
Fair Value	617,010	656,358
Mortgage-backed securities
Available-for-Sale
Total	4,686,448	4,986,354
Unrealized Gains	17	976
Unrealized Losses	(823,760)	(617,428)
Basis adjustments	0	0
Fair Value	3,862,705	4,369,902
Held-to-Maturity
Amortized Cost	1,043,585	1,106,817
Unrealized Gains	0	0
Unrealized Losses	(195,933)	(123,854)
Basis adjustments	0	0
Fair Value	847,652	982,963
States and political subdivisions
Available-for-Sale
Total	592,495	688,159
Unrealized Gains	7	1,789
Unrealized Losses	(47,058)	(26,096)
Basis adjustments	(500)	0
Fair Value	544,944	663,852
Held-to-Maturity
Amortized Cost	1,160,256	1,163,312
Unrealized Gains	0	221
Unrealized Losses	(231,147)	(159,022)
Basis adjustments	0	0
Fair Value	929,109	1,004,511
Pooled trust preferred securities
Available-for-Sale
Total	13,794	13,783
Unrealized Gains	0	0
Unrealized Losses	(2,669)	(2,972)
Basis adjustments	0	0
Fair Value	11,125	10,811
Other securities
Available-for-Sale
Total	335,292	379,423
Unrealized Gains	112	258
Unrealized Losses	(30,443)	(26,541)
Basis adjustments	0	0
Fair Value	$ 304,961	$ 353,140

Debt Securities, Available-For-Sale, Accumulated Basis Adjustments, Before Tax

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