Securities - Available-for-Sale Securities Held by Company (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,599,894	$ 1,293,239
Gross Unrealized Gains	30,773	10,466
Gross Unrealized Losses	3,474	5,118
Fair Value	1,627,193	1,298,587
U.S. Treasuries
Debt Securities, Available-for-sale [Line Items]
Amortized Cost		1,840
Gross Unrealized Gains		0
Gross Unrealized Losses		0
Fair Value		1,840
Obligations of U.S. Government sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	599,652	367,551
Gross Unrealized Gains	9,820	5,021
Gross Unrealized Losses	1,992	84
Fair Value	607,480	372,488
Obligations of U.S. states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	126,642	96,668
Gross Unrealized Gains	3,144	1,178
Gross Unrealized Losses	40	61
Fair Value	129,746	97,785
U.S. Government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	179,538	164,643
Gross Unrealized Gains	3,216	1,327
Gross Unrealized Losses	646	1,519
Fair Value	182,108	164,451
U.S. Government sponsored entities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	691,562	660,037
Gross Unrealized Gains	14,593	2,940
Gross Unrealized Losses	675	3,387
Fair Value	705,480	659,590
U.S. corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,500	2,500
Gross Unrealized Gains	0	0
Gross Unrealized Losses	121	67
Fair Value	$ 2,379	$ 2,433

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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