Available for Sale Securities - Amortized Cost and Estimated Fair Value of Investments Classified as Available for Sale by Contractual Maturity (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 680,497	$ 274,457
Estimated Fair Value	716,561	301,049
Bonds and notes
Schedule of Available-for-sale Securities [Line Items]
Due within one year, Amortized Cost	552,847
Fixed maturities investments excluding mortgage-backed and asset-backed securities, Amortized Cost	552,847
Mortgage-backed and asset-backed securities, Amortized Cost	75,075
Amortized Cost	627,922	221,440
Due within one year, Estimated Fair Value	552,805
Fixed maturities investments excluding mortgage-backed and asset-backed securities, Estimated Fair Value	552,805
Mortgage-backed and asset-backed securities, Estimated Fair Value	75,270
Estimated Fair Value	$ 628,075	$ 221,624