Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Debt securities available-for-sale
Available for sale, amortized cost, total	$ 1,261,879	$ 940,765
Available for sale, gross unrealized gains	28,634	8,764
Available for sale, gross unrealized losses	(3,655)	(2,674)
Available for sale, fair value	1,286,858	946,855
Debt securities held-to-maturity
Held to maturity, amortized cost, total	207,376	261,369
Held to maturity, gross unrealized gains	8,642	2,717
Held to maturity, gross unrealized losses	(18)	(1,873)
Held to maturity, fair value	216,000	262,213
U.S. Treasury securities and obligations of U.S. government agencies
Debt securities available-for-sale
Available for sale, amortized cost, total	8,518	9,914
Available for sale, gross unrealized gains	534	204
Available for sale, gross unrealized losses	(1)	(4)
Available for sale, fair value	9,051	10,114
Mortgage-backed securities and collateralized mortgage obligations of U.S. government-sponsored entities
Debt securities available-for-sale
Available for sale, amortized cost, total	916,231	604,934
Available for sale, gross unrealized gains	23,676	5,784
Available for sale, gross unrealized losses	(646)	(1,511)
Available for sale, fair value	939,261	609,207
Debt securities held-to-maturity
Held to maturity, amortized cost, total	207,376	261,369
Held to maturity, gross unrealized gains	8,642	2,717
Held to maturity, gross unrealized losses	(18)	(1,873)
Held to maturity, fair value	216,000	262,213
Private mortgage-backed securities and collateralized mortgage obligations
Debt securities available-for-sale
Available for sale, amortized cost, total	98,984	56,005
Available for sale, gross unrealized gains	2,099	1,561
Available for sale, gross unrealized losses	(298)	(5)
Available for sale, fair value	100,785	57,561
Debt securities held-to-maturity
Held to maturity, fair value	0
Collateralized loan obligations
Debt securities available-for-sale
Available for sale, amortized cost, total	203,834	239,364
Available for sale, gross unrealized gains	157	7
Available for sale, gross unrealized losses	(2,613)	(1,153)
Available for sale, fair value	201,378	238,218
Debt securities held-to-maturity
Held to maturity, fair value	0
Obligations of state and political subdivisions
Debt securities available-for-sale
Available for sale, amortized cost, total	34,312	30,548
Available for sale, gross unrealized gains	2,168	1,208
Available for sale, gross unrealized losses	(97)	(1)
Available for sale, fair value	$ 36,383	$ 31,755

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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Amount of accumulated unrecognized gain on investment in debt security measured at amortized cost (held-to-maturity).

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Amount, before other-than-temporary impairment (OTTI) accretion, of investment in debt securities measured at amortized cost (held-to-maturity).

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Fair value of investment in debt security measured at amortized cost (held-to-maturity).

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