Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held for Investment (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017
Debt securities available for sale
Available for sale, amortized cost, total	$ 976,182	$ 953,720
Available for sale, gross unrealized gains	1,731	4,202
Available for sale, gross unrealized losses	(23,007)	(8,462)
Available for sale, fair value	954,906	949,460
Debt securities held to maturity
Held to maturity, amortized cost, total	382,137	416,863
Held to maturity, gross unrealized gains	298	1,819
Held to maturity, gross unrealized losses	(11,648)	(4,212)
Held to maturity, fair value	370,787	414,470
U.S. Treasury securities and obligations of U.S. Government Entities
Debt securities available for sale
Available for sale, amortized cost, total	8,372	9,475
Available for sale, gross unrealized gains	137	274
Available for sale, gross unrealized losses	(44)	(5)
Available for sale, fair value	8,465	9,744
Mortgage-backed securities and collateralized mortgage obligations of U.S. Government Sponsored Entities
Debt securities available for sale
Available for sale, amortized cost, total	616,152	560,396
Available for sale, gross unrealized gains	239	1,163
Available for sale, gross unrealized losses	(21,510)	(8,034)
Available for sale, fair value	594,881	553,525
Debt securities held to maturity
Held to maturity, amortized cost, total	324,410	353,541
Held to maturity, gross unrealized gains	0	802
Held to maturity, gross unrealized losses	(11,551)	(4,159)
Held to maturity, fair value	312,859	350,184
Private mortgage-backed securities and collateralized mortgage obligations
Debt securities available for sale
Available for sale, amortized cost, total	80,618	75,152
Available for sale, gross unrealized gains	918	1,154
Available for sale, gross unrealized losses	(404)	(285)
Available for sale, fair value	81,132	76,021
Debt securities held to maturity
Held to maturity, amortized cost, total	22,127	22,799
Held to maturity, gross unrealized gains	241	714
Held to maturity, gross unrealized losses	(96)	(53)
Held to maturity, fair value	22,272	23,460
Collateralized loan obligations
Debt securities available for sale
Available for sale, amortized cost, total	226,549	263,579
Available for sale, gross unrealized gains	98	798
Available for sale, gross unrealized losses	(504)	(68)
Available for sale, fair value	226,143	264,309
Debt securities held to maturity
Held to maturity, amortized cost, total	35,600	40,523
Held to maturity, gross unrealized gains	57	303
Held to maturity, gross unrealized losses	(1)	0
Held to maturity, fair value	35,656	40,826
Obligations of state and political subdivisions
Debt securities available for sale
Available for sale, amortized cost, total	44,491	45,118
Available for sale, gross unrealized gains	339	813
Available for sale, gross unrealized losses	(545)	(70)
Available for sale, fair value	$ 44,285	$ 45,861