Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Amortized cost and fair value of securities available for sale
Amortized Cost	$ 1,322,998	$ 1,276,871
Gross Unrealized Gains	41,915	18,620
Gross Unrealized Losses	(5,784)	(4,878)
Allowance for Credit Losses	0
Fair Value	1,359,129	1,290,613
U.S. Treasury securities
Amortized cost and fair value of securities available for sale
Amortized Cost	7,605	498
Gross Unrealized Gains	67	1
Gross Unrealized Losses	0	0
Allowance for Credit Losses	0
Fair Value	7,672	499
Obligations of other U.S. Government agencies and corporations
Amortized cost and fair value of securities available for sale
Amortized Cost	2,514	2,518
Gross Unrealized Gains	31	16
Gross Unrealized Losses	0	(3)
Allowance for Credit Losses	0
Fair Value	2,545	2,531
Obligations of states and political subdivisions
Amortized cost and fair value of securities available for sale
Amortized Cost	256,974	218,362
Gross Unrealized Gains	6,399	5,134
Gross Unrealized Losses	(1,872)	(365)
Allowance for Credit Losses	0
Fair Value	261,501	223,131
Trust preferred securities
Amortized cost and fair value of securities available for sale
Amortized Cost	12,091	12,153
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(3,487)	(2,167)
Allowance for Credit Losses	0
Fair Value	8,604	9,986
Other debt securities
Amortized cost and fair value of securities available for sale
Amortized Cost	56,380	55,364
Gross Unrealized Gains	1,677	1,133
Gross Unrealized Losses	(245)	(132)
Allowance for Credit Losses	0
Fair Value	57,812	56,365
Government agency mortgage backed securities \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	709,117	708,970
Gross Unrealized Gains	24,296	8,951
Gross Unrealized Losses	0	(1,816)
Allowance for Credit Losses	0
Fair Value	733,413	716,105
Government agency mortgage backed securities \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	32,625	30,372
Gross Unrealized Gains	1,967	659
Gross Unrealized Losses	(2)	(24)
Allowance for Credit Losses	0
Fair Value	34,590	31,007
Government agency collateralized mortgage obligations \| Residential mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	159,940	172,178
Gross Unrealized Gains	4,653	1,322
Gross Unrealized Losses	0	(262)
Allowance for Credit Losses	0
Fair Value	164,593	173,238
Government agency collateralized mortgage obligations \| Commercial mortgage backed securities:
Amortized cost and fair value of securities available for sale
Amortized Cost	85,752	76,456
Gross Unrealized Gains	2,825	1,404
Gross Unrealized Losses	(178)	(109)
Allowance for Credit Losses	0
Fair Value	$ 88,399	$ 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, before tax, of unrealized loss 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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