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Investment Securities - Summary of Investment Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Held-to-maturity
Amortized Cost	$ 335,946	$ 40,927
Allowance for Credit Losses	(2,915)	0
Net Carrying Amount	333,031	40,927
Gross Unrealized Gains	8,924	987
Gross Unrealized (Losses)	(30)	(59)
Estimated Fair Value	341,925	41,855
Available-for-sale
Available-for-sale, amortized cost	3,397,043	3,263,151
Allowance for Credit Losses	(312)	0
Gross Unrealized Gains	79,663	31,740
Gross Unrealized (Losses)	(2,796)	(6,548)
Estimated Fair Value	3,473,598	3,288,343
U.S. Treasury
Available-for-sale
Available-for-sale, amortized cost		449,729
Allowance for Credit Losses		0
Gross Unrealized Gains		112
Gross Unrealized (Losses)		(112)
Estimated Fair Value		449,729
U.S. Government agencies
Available-for-sale
Available-for-sale, amortized cost	477,693	194,207
Allowance for Credit Losses	0	0
Gross Unrealized Gains	844	1,313
Gross Unrealized (Losses)	(1,300)	(1,271)
Estimated Fair Value	477,237	194,249
Mortgage-backed securities
Held-to-maturity
Amortized Cost	22,354	10,796
Allowance for Credit Losses	0	0
Net Carrying Amount	22,354	10,796
Gross Unrealized Gains	683	71
Gross Unrealized (Losses)	0	(59)
Estimated Fair Value	23,037	10,808
Available-for-sale
Available-for-sale, amortized cost	1,374,769	1,738,584
Allowance for Credit Losses	0	0
Gross Unrealized Gains	21,261	8,510
Gross Unrealized (Losses)	(1,094)	(4,149)
Estimated Fair Value	1,394,936	1,742,945
State and political subdivisions
Held-to-maturity
Amortized Cost	312,416	27,082
Allowance for Credit Losses	(2,307)	0
Net Carrying Amount	310,109	27,082
Gross Unrealized Gains	8,148	849
Gross Unrealized (Losses)	(30)	0
Estimated Fair Value	318,227	27,931
Available-for-sale
Available-for-sale, amortized cost	1,416,136	860,539
Allowance for Credit Losses	(217)	0
Gross Unrealized Gains	55,111	20,983
Gross Unrealized (Losses)	(307)	(998)
Estimated Fair Value	1,470,723	880,524
Other securities
Held-to-maturity
Amortized Cost	1,176	3,049
Allowance for Credit Losses	(608)	0
Net Carrying Amount	568	3,049
Gross Unrealized Gains	93	67
Gross Unrealized (Losses)	0	0
Estimated Fair Value	661	3,116
Available-for-sale
Available-for-sale, amortized cost	128,445	20,092
Allowance for Credit Losses	(95)	0
Gross Unrealized Gains	2,447	822
Gross Unrealized (Losses)	(95)	(18)
Estimated Fair Value	$ 130,702	$ 20,896

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