Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2022	Dec. 31, 2021	Mar. 31, 2021	Dec. 31, 2020
Held-to-maturity
Amortized Cost	$ 1,558,202	$ 1,530,500
Allowance for credit losses on HTM	(1,377)	(1,279)	$ (1,618)	$ (2,915)
Net Carrying Amount	1,556,825	1,529,221
Gross Unrealized Gains	385	6,398
Gross Unrealized (Losses)	(250,152)	(18,241)
Estimated Fair Value	1,307,058	1,517,378
Available-for-sale
Amortized Cost	7,070,582	7,130,861
Allowance for credit losses on AFS	0	0	(2,454)	(312)
Gross Unrealized Gains	5,413	44,005
Gross Unrealized (Losses)	(435,926)	(61,321)
Estimated Fair Value	6,640,069	7,113,545
U.S. Treasury
Available-for-sale
Amortized Cost	0	300
Allowance for credit losses on AFS	0	0
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	0	0
Estimated Fair Value	0	300
U.S. Government agencies
Held-to-maturity
Amortized Cost	232,670	232,609
Allowance for credit losses on HTM	0	0
Net Carrying Amount	232,670	232,609
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(31,510)	(7,914)
Estimated Fair Value	201,160	224,695
Available-for-sale
Amortized Cost	367,730	374,754
Allowance for credit losses on AFS	0	0
Gross Unrealized Gains	239	495
Gross Unrealized (Losses)	(34,738)	(10,608)
Estimated Fair Value	333,231	364,641
Mortgage-backed securities
Held-to-maturity
Amortized Cost	112,496	70,342
Allowance for credit losses on HTM	0	0
Net Carrying Amount	112,496	70,342
Gross Unrealized Gains	1	232
Gross Unrealized (Losses)	(7,552)	(1,425)
Estimated Fair Value	104,945	69,149
Available-for-sale
Amortized Cost	4,378,489	4,485,548
Allowance for credit losses on AFS	0	0
Gross Unrealized Gains	138	6,307
Gross Unrealized (Losses)	(212,519)	(43,239)
Estimated Fair Value	4,166,108	4,448,616
State and political subdivisions
Held-to-maturity
Amortized Cost	1,195,744	1,210,248
Allowance for credit losses on HTM	(1,285)	(1,197)	(1,042)	(2,307)
Net Carrying Amount	1,194,459	1,209,051
Gross Unrealized Gains	384	6,166
Gross Unrealized (Losses)	(209,191)	(8,462)
Estimated Fair Value	985,652	1,206,755
Available-for-sale
Amortized Cost	1,812,534	1,791,097
Allowance for credit losses on AFS	0	0	(64)	(217)
Gross Unrealized Gains	2,727	30,556
Gross Unrealized (Losses)	(161,567)	(1,995)
Estimated Fair Value	1,653,694	1,819,658
Other securities
Held-to-maturity
Amortized Cost	17,292	17,301
Allowance for credit losses on HTM	(92)	(82)	(576)	(608)
Net Carrying Amount	17,200	17,219
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(1,899)	(440)
Estimated Fair Value	15,301	16,779
Available-for-sale
Amortized Cost	511,829	479,162
Allowance for credit losses on AFS	0	0	$ (2,390)	$ (95)
Gross Unrealized Gains	2,309	6,647
Gross Unrealized (Losses)	(27,102)	(5,479)
Estimated Fair Value	$ 487,036	$ 480,330

Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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