Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Amortized cost and fair value of securities available for sale
Amortized Cost	$ 2,532,767	$ 1,301,973
Gross Unrealized Gains	26,502	44,699
Gross Unrealized Losses	(14,626)	(3,215)
Fair Value	2,544,643	1,343,457
U.S. Treasury securities
Amortized cost and fair value of securities available for sale
Amortized Cost	3,014	7,047
Gross Unrealized Gains	10	32
Gross Unrealized Losses	0	0
Fair Value	3,024	7,079
Obligations of other U.S. Government agencies and corporations
Amortized cost and fair value of securities available for sale
Amortized Cost		1,003
Gross Unrealized Gains		6
Gross Unrealized Losses		0
Fair Value		1,009
Obligations of states and political subdivisions
Amortized cost and fair value of securities available for sale
Amortized Cost	372,311	291,231
Gross Unrealized Gains	9,096	14,015
Gross Unrealized Losses	(2,426)	(45)
Fair Value	378,981	305,201
Trust preferred securities
Amortized cost and fair value of securities available for sale
Amortized Cost		12,013
Gross Unrealized Gains		0
Gross Unrealized Losses		(3,001)
Fair Value		9,012
Other debt securities
Amortized cost and fair value of securities available for sale
Amortized Cost	51,321	62,771
Gross Unrealized Gains	2,646	2,909
Gross Unrealized Losses	0	(73)
Fair Value	53,967	65,607
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	935,233	581,105
Gross Unrealized Gains	11,694	21,564
Gross Unrealized Losses	(3,689)	(23)
Fair Value	943,238	602,646
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	16,650	29,053
Gross Unrealized Gains	515	1,235
Gross Unrealized Losses	(1)	(1)
Fair Value	17,164	30,287
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	962,684	218,373
Gross Unrealized Gains	1,225	1,946
Gross Unrealized Losses	(6,172)	(51)
Fair Value	957,737	220,268
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	191,554	99,377
Gross Unrealized Gains	1,316	2,992
Gross Unrealized Losses	(2,338)	(21)
Fair Value	$ 190,532	$ 102,348

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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Amortized cost of 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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