Securities - Amortized Cost and Fair Value of Securities by Contractual Maturity (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held to maturity, amortized cost, total	$ 367,387	$ 416,863
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held to maturity, fair value, total	353,919	414,470
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available for sale, amortized cost, total	948,030	953,720
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available for sale, fair value, total	923,206	949,460
Mortgage-backed securities and collateralized mortgage obligations of U.S. Government Sponsored Entities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held to maturity, amortized cost, total	313,667
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held to maturity, fair value, total	300,110	350,184
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available for sale, amortized cost, total	597,689	560,396
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available for sale, fair value, total	573,182	553,525
Private mortgage backed securities
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held to maturity, amortized cost, total	21,720
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held to maturity, fair value, total	21,770	23,460
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available for sale, amortized cost, total	75,485	75,152
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available for sale, fair value, total	76,092	$ 76,021
Collateralized loan obligations
Held-to-maturity Securities, Debt Maturities, Net Carrying Amount [Abstract]
Held to maturity, amortized cost, due in less than one year	0
Held to maturity, amortized cost, due after one year through five years	0
Held to maturity, amortized cost, due after five years through ten years	32,000
Held to maturity, amortized cost, due after ten years	0
Held to maturity, amortized cost, total	32,000
Held-to-maturity Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Held to maturity, fair value, due in less than one year	0
Held to maturity, fair value, due after one year through five years	0
Held to maturity, fair value, due after five years through ten years	32,039
Held to maturity, fair value, due after ten years	0
Held to maturity, fair value, total	32,039
Available-for-sale Securities, Debt Maturities, Amortized Cost Basis, Fiscal Year Maturity [Abstract]
Available for sale, amortized cost, due in less than one year	13,832
Available for sale, amortized cost, due after one year through five years	79,637
Available for sale, amortized cost, due after five years through ten years	177,911
Available for sale, amortized cost, due after ten years	3,476
Available for sale, amortized cost, total	274,856
Available-for-sale Securities, Debt Maturities, Fair Value, Fiscal Year Maturity [Abstract]
Available for sale, fair value, due in less than one year	13,781
Available for sale, fair value, due after one year through five years	79,582
Available for sale, fair value, due after five years through ten years	177,196
Available for sale, fair value, due after ten years	3,373
Available for sale, fair value, total	$ 273,932