Securities - Amortized Cost and Fair Value of Securities Available for Sale and Held for Investment (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Debt securities available for sale
Available for sale, amortized cost, total	$ 948,030	$ 953,720
Available for sale, gross unrealized gains	1,590	4,202
Available for sale, gross unrealized losses	(26,414)	(8,462)
Available for sale, fair value	923,206	949,460
Debt securities held to maturity
Held to maturity, amortized cost, total	367,387	416,863
Held to maturity, gross unrealized gains	239	1,819
Held to maturity, gross unrealized losses	(13,707)	(4,212)
Held to maturity, fair value	353,919	414,470
U.S. Treasury securities and obligations of U.S. Government Entities
Debt securities available for sale
Available for sale, amortized cost, total	7,486	9,475
Available for sale, gross unrealized gains	114	274
Available for sale, gross unrealized losses	(57)	(5)
Available for sale, fair value	7,543	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	597,689	560,396
Available for sale, gross unrealized gains	153	1,163
Available for sale, gross unrealized losses	(24,660)	(8,034)
Available for sale, fair value	573,182	553,525
Debt securities held to maturity
Held to maturity, amortized cost, total	313,667	353,541
Held to maturity, gross unrealized gains	0	802
Held to maturity, gross unrealized losses	(13,557)	(4,159)
Held to maturity, fair value	300,110	350,184
Private mortgage-backed securities and collateralized mortgage obligations
Debt securities available for sale
Available for sale, amortized cost, total	75,485	75,152
Available for sale, gross unrealized gains	925	1,154
Available for sale, gross unrealized losses	(318)	(285)
Available for sale, fair value	76,092	76,021
Debt securities held to maturity
Held to maturity, amortized cost, total	21,720	22,799
Held to maturity, gross unrealized gains	181	714
Held to maturity, gross unrealized losses	(131)	(53)
Held to maturity, fair value	21,770	23,460
Collateralized loan obligations
Debt securities available for sale
Available for sale, amortized cost, total	223,419	263,579
Available for sale, gross unrealized gains	137	798
Available for sale, gross unrealized losses	(566)	(68)
Available for sale, fair value	222,990	264,309
Debt securities held to maturity
Held to maturity, amortized cost, total	32,000	40,523
Held to maturity, gross unrealized gains	58	303
Held to maturity, gross unrealized losses	(19)	0
Held to maturity, fair value	32,039	40,826
Obligations of state and political subdivisions
Debt securities available for sale
Available for sale, amortized cost, total	43,951	45,118
Available for sale, gross unrealized gains	261	813
Available for sale, gross unrealized losses	(813)	(70)
Available for sale, fair value	$ 43,399	$ 45,861