Investments - Maturity Table (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 247,607	$ 211,406
Mortgage-backed securities	244,088	$ 208,603
Maturity 2038
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	272
Mortgage-backed securities	293
Maturity 2039
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,406
Mortgage-backed securities	3,606
Maturity 2042
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,993
Mortgage-backed securities	10,487
Maturity 2043
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	37,438
Mortgage-backed securities	37,534
Maturity 2044
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	28,301
Mortgage-backed securities	28,206
Maturity 2045
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	41,082
Mortgage-backed securities	40,484
Maturity 2046
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	48,785
Mortgage-backed securities	47,771
Maturity 2047
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	77,330
Mortgage-backed securities	$ 75,707