Schedule I Summary of Investments, Other than Investments in Related Parties (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	$ 4,005,642	$ 3,562,864
HTM securities, Amortized Cost	752,212	607,843
Other Investments, Amortized Cost	10,057	10,816
Total investments, Original or amortized cost	4,767,911	4,181,523
Available-for-sale fixed maturities, Fair value	3,971,666	3,508,088
HTM securities, Fair value	766,135	598,975
Other investments, Fair value	13,060	11,812
Total investments, Fair value	4,750,861	4,118,875
Amount at Which Shown in the Balance Sheet	4,736,938
U.S. treasury bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	5,186	5,714
Available-for-sale fixed maturities, Fair value	5,413	6,010
Amount at Which Shown in the Balance Sheet	5,413
U.S. agency bonds – mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	1,720,436	1,471,782
Available-for-sale fixed maturities, Fair value	1,716,038	1,476,991
Amount at Which Shown in the Balance Sheet	1,716,038
U.S. agency bonds – other
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	18,082	23,734
Available-for-sale fixed maturities, Fair value	18,102	24,311
Amount at Which Shown in the Balance Sheet	18,102
Non-U.S. government and supranational bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	35,158	35,128
Available-for-sale fixed maturities, Fair value	29,934	30,544
Amount at Which Shown in the Balance Sheet	29,934
Asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	217,232
Available-for-sale fixed maturities, Fair value	220,876
Amount at Which Shown in the Balance Sheet	220,876
Corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	1,947,347	1,798,610
HTM securities, Amortized Cost	752,212	607,843
Available-for-sale fixed maturities, Fair value	1,916,205	1,739,668
HTM securities, Fair value	766,135	$ 598,975
Amount at Which Shown in the Balance Sheet	1,916,205
Municipal bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	62,201
Available-for-sale fixed maturities, Fair value	65,098
Amount at Which Shown in the Balance Sheet	65,098
Total AFS fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fixed Maturities, available-for-sale, Amortized cost	4,005,642
Available-for-sale fixed maturities, Fair value	3,971,666
Amount at Which Shown in the Balance Sheet	3,971,666
HTM Corporate bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
HTM securities, Amortized Cost	752,212
HTM securities, Fair value	766,135
Amount at Which Shown in the Balance Sheet	752,212
HTM fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
HTM securities, Amortized Cost	752,212
HTM securities, Fair value	766,135
Amount at Which Shown in the Balance Sheet	752,212
Other investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Other Investments, Amortized Cost	10,057
Other investments, Fair value	13,060
Amount at Which Shown in the Balance Sheet	$ 13,060