Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held for Investment (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Available-for-Sale Debt Securities
Available for sale, amortized cost, total	$ 1,371,539	$ 940,765
Available for sale, gross unrealized gains	28,716	8,764
Available for sale, gross unrealized losses	(2,098)	(2,674)
Available for sale, fair value	1,398,157	946,855
Held-to-Maturity Debt Securities
Held to maturity, amortized cost	184,484	261,369
Held to maturity, gross unrealized gains	7,818	2,717
Held to maturity, gross unrealized losses	(123)	(1,873)
Held to maturity, fair value	192,179	262,213
U.S. Treasury securities and obligations of U.S. government agencies
Available-for-Sale Debt Securities
Available for sale, amortized cost, total	8,250	9,914
Available for sale, gross unrealized gains	528	204
Available for sale, gross unrealized losses	(1)	(4)
Available for sale, fair value	8,777	10,114
Mortgage-backed securities and collateralized mortgage obligations of U.S. government-sponsored entities
Available-for-Sale Debt Securities
Available for sale, amortized cost, total	1,038,437	604,934
Available for sale, gross unrealized gains	23,457	5,784
Available for sale, gross unrealized losses	(1,240)	(1,511)
Available for sale, fair value	1,060,654	609,207
Held-to-Maturity Debt Securities
Held to maturity, amortized cost	184,484	261,369
Held to maturity, gross unrealized gains	7,818	2,717
Held to maturity, gross unrealized losses	(123)	(1,873)
Held to maturity, fair value	192,179	262,213
Private mortgage-backed securities and collateralized mortgage obligations
Available-for-Sale Debt Securities
Available for sale, amortized cost, total	89,284	56,005
Available for sale, gross unrealized gains	2,131	1,561
Available for sale, gross unrealized losses	(210)	(5)
Available for sale, fair value	91,205	57,561
Held-to-Maturity Debt Securities
Held to maturity, fair value	0
Collateralized loan obligations
Available-for-Sale Debt Securities
Available for sale, amortized cost, total	202,563	239,364
Available for sale, gross unrealized gains	279	7
Available for sale, gross unrealized losses	(647)	(1,153)
Available for sale, fair value	202,195	238,218
Held-to-Maturity Debt Securities
Held to maturity, fair value	0
Obligations of state and political subdivisions
Available-for-Sale Debt Securities
Available for sale, amortized cost, total	33,005	30,548
Available for sale, gross unrealized gains	2,321	1,208
Available for sale, gross unrealized losses	0	(1)
Available for sale, fair value	$ 35,326	$ 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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