Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Amortized cost and fair value of securities available for sale
Amortized Cost	$ 1,253,273	$ 1,276,871
Gross Unrealized Gains	44,618	18,620
Gross Unrealized Losses	(4,503)	(4,878)
Allowance for Credit Losses	0
Fair Value	1,293,388	1,290,613
U.S. Treasury securities
Amortized cost and fair value of securities available for sale
Amortized Cost	7,067	498
Gross Unrealized Gains	45	1
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0
Fair Value	7,112	499
Obligations of other U.S. Government agencies and corporations
Amortized cost and fair value of securities available for sale
Amortized Cost	1,505	2,518
Gross Unrealized Gains	10	16
Gross Unrealized Losses	0	(3)
Allowance for Credit Losses	0
Fair Value	1,515	2,531
Obligations of states and political subdivisions
Amortized cost and fair value of securities available for sale
Amortized Cost	267,067	218,362
Gross Unrealized Gains	11,224	5,134
Gross Unrealized Losses	(651)	(365)
Allowance for Credit Losses	0
Fair Value	277,640	223,131
Trust preferred securities
Amortized cost and fair value of securities available for sale
Amortized Cost	12,042	12,153
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3,550)	(2,167)
Allowance for Credit Losses	0
Fair Value	8,492	9,986
Other debt securities
Amortized cost and fair value of securities available for sale
Amortized Cost	62,495	55,364
Gross Unrealized Gains	2,935	1,133
Gross Unrealized Losses	(131)	(132)
Allowance for Credit Losses	0
Fair Value	65,299	56,365
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	649,383	708,970
Gross Unrealized Gains	23,601	8,951
Gross Unrealized Losses	(64)	(1,816)
Allowance for Credit Losses	0
Fair Value	672,920	716,105
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	29,198	30,372
Gross Unrealized Gains	1,316	659
Gross Unrealized Losses	(1)	(24)
Allowance for Credit Losses	0
Fair Value	30,513	31,007
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	133,993	172,178
Gross Unrealized Gains	2,181	1,322
Gross Unrealized Losses	(29)	(262)
Allowance for Credit Losses	0
Fair Value	136,145	173,238
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	90,523	76,456
Gross Unrealized Gains	3,306	1,404
Gross Unrealized Losses	(77)	(109)
Allowance for Credit Losses	0
Fair Value	$ 93,752	$ 77,751

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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Amount of allowance for credit loss 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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