Investments - Schedule of Available-for-sale and Held-to-Maturity Securities Reconciliation (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Jan. 01, 2020	Dec. 31, 2019
Debt Securities, Held-to-maturity [Abstract]
Amortized Cost	$ 327,694		$ 181,166
Gross Unrealized Gains	11,873		973
Gross Unrealized Losses	(148)		(200)
Allowance for Credit Losses	645	$ 0
Fair Value	338,774		181,939
Total securities held-to-maturity, net	327,049		181,166
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	968,598		1,115,323
Gross Unrealized Gains	37,415		21,548
Gross Unrealized Losses	(587)		(1,554)
Allowance for Credit Losses	0
Fair Value	1,005,426		1,135,317
Obligations of U.S. Government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	9,972		9,954
Gross Unrealized Gains	228		92
Gross Unrealized Losses	0		0
Allowance for Credit Losses	0
Fair Value	10,200		10,046
Obligations of states and political subdivisions
Debt Securities, Held-to-maturity [Abstract]
Amortized Cost	95,497		11,704
Gross Unrealized Gains	1,038		170
Gross Unrealized Losses	0		0
Allowance for Credit Losses	16
Fair Value	96,519		11,874
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	355,036		207,269
Gross Unrealized Gains	11,625		6,118
Gross Unrealized Losses	(539)		(363)
Allowance for Credit Losses	0
Fair Value	366,122		213,024
Agency mortgage-backed securities
Debt Securities, Held-to-maturity [Abstract]
Amortized Cost	109,954		46,346
Gross Unrealized Gains	2,468		675
Gross Unrealized Losses	(148)		0
Allowance for Credit Losses	0
Fair Value	112,274		47,021
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	581,111		888,129
Gross Unrealized Gains	24,884		15,083
Gross Unrealized Losses	(46)		(1,191)
Allowance for Credit Losses	0
Fair Value	605,949		902,021
US Treasury Bill Securities [Member]
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	10,977		9,971
Gross Unrealized Gains	544		255
Gross Unrealized Losses	0		0
Allowance for Credit Losses	0
Fair Value	11,521		10,226
Corporate Debt Securities [Member]
Debt Securities, Held-to-maturity [Abstract]
Amortized Cost	122,243		123,116
Gross Unrealized Gains	8,367		128
Gross Unrealized Losses	0		(200)
Allowance for Credit Losses	629
Fair Value	129,981		$ 123,044
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	11,502
Gross Unrealized Gains	134
Gross Unrealized Losses	(2)
Allowance for Credit Losses	0
Fair Value	$ 11,634

Debt Securities, Held-To-Maturity, Net Of Allowance For Credit Losses

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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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