Investment Securities - Amortized Cost and Fair Value of Securities Available for Sale (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,161,098	$ 1,001,034
Gross Unrealized Gains	14,639	5,618
Gross Unrealized Losses	(773)	(668)
Fair Value	1,174,964	1,005,984
U.S. Treasury securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	500,095	500,066
Gross Unrealized Gains	5	35
Gross Unrealized Losses	0	0
Fair Value	500,100	500,101
Obligations of the U.S. government and agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	92,449	102,179
Gross Unrealized Gains	868	193
Gross Unrealized Losses	(219)	(352)
Fair Value	93,098	102,020
Obligations of state and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	2,149	5,366
Gross Unrealized Gains	22	13
Gross Unrealized Losses	0	0
Fair Value	2,171	5,379
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	441,575	360,977
Gross Unrealized Gains	12,739	5,182
Gross Unrealized Losses	(457)	(157)
Fair Value	453,857	366,002
Collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	18,680	31,796
Gross Unrealized Gains	583	195
Gross Unrealized Losses	0	(159)
Fair Value	19,263	31,832
Collateralized loan obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	94,500	0
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(97)	0
Fair Value	94,404	0
Corporate bonds
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	11,000	0
Gross Unrealized Gains	421	0
Gross Unrealized Losses	0	0
Fair Value	11,421	0
Other investment securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	650	650
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 650	$ 650

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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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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Amount 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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