Schedule I - Summary of investments (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	$ 8,761,087
Fair value	8,744,912
Amount at which shown on the Balance Sheet	8,744,912
Total fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	5,584,599
Fair value	5,543,030
Amount at which shown on the Balance Sheet	5,543,030
U.S. government and government agency
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	809,392
Fair value	804,126
Amount at which shown on the Balance Sheet	804,126
Non-U.S. government and government agency
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	245,651
Fair value	240,791
Amount at which shown on the Balance Sheet	240,791
U.S. states, municipalities and political subdivisions
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	271,742
Fair value	271,830
Amount at which shown on the Balance Sheet	271,830
Agency residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	684,490
Fair value	679,595
Amount at which shown on the Balance Sheet	679,595
Non-agency residential mortgage-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	15,858
Fair value	15,477
Amount at which shown on the Balance Sheet	15,477
U.S. corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	1,540,036
Fair value	1,534,508
Amount at which shown on the Balance Sheet	1,534,508
Non-U.S. corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	418,520
Fair value	410,227
Amount at which shown on the Balance Sheet	410,227
Bank loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	579,121
Fair value	570,399
Amount at which shown on the Balance Sheet	570,399
Catastrophe bonds
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	157,486
Fair value	158,331
Amount at which shown on the Balance Sheet	158,331
Asset-backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	528,563
Fair value	526,814
Amount at which shown on the Balance Sheet	526,814
Commercial mortgage backed securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	333,740
Fair value	330,932
Amount at which shown on the Balance Sheet	330,932
Total short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	2,796,358
Fair value	2,796,170
Amount at which shown on the Balance Sheet	2,796,170
Total other investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized cost	380,130
Fair value	405,712
Amount at which shown on the Balance Sheet	$ 405,712