Securities - AFS Contractual Maturities (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
AFS, Amortized Cost, Rolling Maturity
Due in one year or less	$ 101,940
Due after one year through five years	95,039
Due after five years through ten years	27,646
Due after ten years	19,550
Total	244,175
Total amortized cost	819,113	$ 748,255
AFS, Fair Value, Rolling Maturity
Due in one year or less	101,919
Due after one year through five years	94,840
Due after five years through ten years	27,948
Due after ten years	20,111
Total	244,818
Fair Value	806,583	744,319
Residential mortgage-backed Securities
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	301,658
Total amortized cost	301,658	246,707
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	294,268
Fair Value	294,268	243,505
Collateralized mortgage obligations
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	261,406
Total amortized cost	261,406	237,848
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	255,768
Fair Value	255,768	$ 233,812
Commercial mortgage-backed Securities
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	11,874
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	$ 11,729