Investments - Maturity Table (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	$ 211,406	$ 196,402
Mortgage-backed securities	208,603	$ 195,391
Maturity 2038
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	278
Mortgage-backed securities	303
Maturity 2039
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	5,405
Mortgage-backed securities	5,758
Maturity 2042
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	15,344
Mortgage-backed securities	14,621
Maturity 2043
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	46,996
Mortgage-backed securities	47,045
Maturity 2044
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	37,970
Mortgage-backed securities	37,876
Maturity 2045
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	50,814
Mortgage-backed securities	50,084
Maturity 2046
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	39,599
Mortgage-backed securities	38,606
Maturity 2047
Schedule of Available-for-sale Securities [Line Items]
Available-for-sale investments at fair value	15,000
Mortgage-backed securities	$ 14,310