Debt Securities Available for Sale - Securities Available-for-Sale (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,204,001	$ 1,086,143
Gross Unrealized Gains	42,787	14,248
Gross Unrealized (Losses)	(1,488)	(2,055)
Debt securities available for sale, at fair value	1,245,300	1,098,336
U.S. government and agency obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	29,421	42,081
Gross Unrealized Gains	1,247	321
Gross Unrealized (Losses)	0	(16)
Debt securities available for sale, at fair value	30,668	42,386
Mortgage-backed securities and collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,095,120	968,165
Gross Unrealized Gains	39,429	12,981
Gross Unrealized (Losses)	(443)	(1,265)
Debt securities available for sale, at fair value	1,134,106	979,881
Municipal obligations
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,870	2,284
Gross Unrealized Gains	15	1
Gross Unrealized (Losses)	0	(1)
Debt securities available for sale, at fair value	1,885	2,284
Corporate debt securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	72,590	68,613
Gross Unrealized Gains	2,096	945
Gross Unrealized (Losses)	(505)	(378)
Debt securities available for sale, at fair value	74,181	69,180
Trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	5,000	5,000
Gross Unrealized Gains	0	0
Gross Unrealized (Losses)	(540)	(395)
Debt securities available for sale, at fair value	$ 4,460	$ 4,605

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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