Securities - AFS Contractual Maturities (Details) $ in Thousands	Sep. 30, 2017 USD ($)
AFS, Amortized Cost, Rolling Maturity
Due in one year or less	$ 108,593
Due after one year through five years	86,378
Due after five years through ten years	40,798
Due after ten years	25,710
Total	261,479
Total amortized cost	743,528
AFS, Fair Value, Rolling Maturity
Due in one year or less	108,958
Due after one year through five years	88,189
Due after five years through ten years	42,418
Due after ten years	26,702
Total	266,267
Fair Value	744,559
Residential mortgage-backed Securities
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	247,272
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	245,908
Collateralized mortgage obligations
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	222,187
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	219,734
Commercial mortgage-backed Securities
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	12,590
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	$ 12,650