Securities - Contractual Maturities (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Contractual Maturities, Amortized Cost
Within one year	$ 184,928	$ 164,997
After one through five years	198,515	212,905
After ten years	6,200	6,200
Contractual Maturities, Amortized Cost, Single Maturity Date	389,643	384,102
Available-for-sale Securities, Amortized Cost Basis, Total	415,563	410,062
Contractual Maturities, Fair Value
Within one year	184,268	164,321
After one through five years	197,051	211,890
After ten years	6,200	6,200
Contractual Maturities, Fair Value, Single Maturity Date	387,519	382,411
Fair Value	413,457	408,371
Proceeds from the sale of securities available for sale	0
Mortgage-backed - Government-sponsored entity (GSE) - residential
Contractual Maturities, Amortized Cost
Contractual Maturities, Amortized Cost, Not Due at a Single Maturity Date	25,851	25,960
Available-for-sale Securities, Amortized Cost Basis, Total	25,851	25,960
Contractual Maturities, Fair Value
Contractual Maturities, Fair Value, Not Due at a Single Maturity Date	25,851	25,960
Fair Value	25,851	$ 25,960
Equities
Contractual Maturities, Amortized Cost
Contractual Maturities, Amortized Cost, Not Due at a Single Maturity Date	69
Available-for-sale Securities, Amortized Cost Basis, Total	69
Contractual Maturities, Fair Value
Contractual Maturities, Fair Value, Not Due at a Single Maturity Date	87
Fair Value	$ 87