Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2021	Dec. 31, 2020
Debt securities available-for-sale
Available for sale, amortized cost, total	$ 1,539,560	$ 1,371,539
Available for sale, gross unrealized gains	16,445	28,716
Available for sale, gross unrealized losses	(9,850)	(2,098)
Available for sale, fair value	1,546,155	1,398,157
Debt securities held-to-maturity
Held to maturity, amortized cost, total	526,502	184,484
Held to maturity, gross unrealized gains	4,702	7,818
Held to maturity, gross unrealized losses	(12,740)	(123)
Held to maturity, fair value	518,464	192,179
U.S. Treasury securities and obligations of U.S. government agencies
Debt securities available-for-sale
Available for sale, amortized cost, total	6,785	8,250
Available for sale, gross unrealized gains	371	528
Available for sale, gross unrealized losses	(2)	(1)
Available for sale, fair value	7,154	8,777
Mortgage-backed securities and collateralized mortgage obligations of U.S. government-sponsored entities
Debt securities available-for-sale
Available for sale, amortized cost, total	1,147,743	1,038,437
Available for sale, gross unrealized gains	12,378	23,457
Available for sale, gross unrealized losses	(9,450)	(1,240)
Available for sale, fair value	1,150,671	1,060,654
Debt securities held-to-maturity
Held to maturity, amortized cost, total		184,484
Held to maturity, gross unrealized gains		7,818
Held to maturity, gross unrealized losses		(123)
Held to maturity, fair value	518,464	192,179
Private mortgage-backed securities and collateralized mortgage obligations
Debt securities available-for-sale
Available for sale, amortized cost, total	75,754	89,284
Available for sale, gross unrealized gains	1,710	2,131
Available for sale, gross unrealized losses	(236)	(210)
Available for sale, fair value	77,228	91,205
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	277,262	202,563
Available for sale, gross unrealized gains	149	279
Available for sale, gross unrealized losses	(105)	(647)
Available for sale, fair value	277,306	202,195
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	32,016	33,005
Available for sale, gross unrealized gains	1,837	2,321
Available for sale, gross unrealized losses	(57)	0
Available for sale, fair value	33,796	$ 35,326
Mortgage-backed securities and collateralized mortgage obligations of U.S. government-sponsored entities
Debt securities held-to-maturity
Held to maturity, amortized cost, total	526,502
Held to maturity, gross unrealized gains	4,702
Held to maturity, gross unrealized losses	(12,740)
Held to maturity, fair value	$ 518,464

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 excluding accrued interest, after allowance for credit loss, 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 excluding accrued interest, 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 excluding accrued interest, after allowance for credit loss, of investment in debt security measured at amortized cost (held-to-maturity).

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Amount of accumulated unrealized loss on investment in debt security 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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