Securities - AFS Contractual Maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
AFS, Amortized Cost, Rolling Maturity
Due in one year or less	$ 49,477
Due after one year through five years	104,633
Due after five years through ten years	34,663
Due after ten years	22,632
Total	211,405
Total amortized cost	893,500	$ 748,255
AFS, Fair Value, Rolling Maturity
Due in one year or less	49,466
Due after one year through five years	103,861
Due after five years through ten years	34,218
Due after ten years	22,974
Total	210,519
Fair Value	874,496	744,319
Residential mortgage-backed Securities
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	379,759
Total amortized cost	379,759	246,707
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	369,213
Fair Value	369,213	243,505
Collateralized mortgage obligations
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	290,576
Total amortized cost	290,576	237,848
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	283,175
Fair Value	283,175	$ 233,812
Commercial mortgage-backed Securities
AFS, Amortized Cost, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	11,760
AFS, Fair Value, Rolling Maturity
Mortgage-backed securities, collateralized mortgage obligations and commercial mortgage-backed securities	$ 11,589