Schedule I. Summary of Investments Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Market Value	$ 9,316,968
Amount at which shown in the Balance Sheet	9,316,968
U.S. treasuries
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,635,282
Market Value	2,617,894
Amount at which shown in the Balance Sheet	2,617,894
Agencies
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	91,905
Market Value	90,972
Amount at which shown in the Balance Sheet	90,972
Municipal
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	524,559
Market Value	519,069
Amount at which shown in the Balance Sheet	519,069
Non-U.S. government (Sovereign debt)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	342,108
Market Value	333,224
Amount at which shown in the Balance Sheet	333,224
Non-U.S. government-backed corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	137,024
Market Value	133,300
Amount at which shown in the Balance Sheet	133,300
Corporate
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,868,125
Market Value	1,877,243
Amount at which shown in the Balance Sheet	1,877,243
Agency mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	471,235
Market Value	462,493
Amount at which shown in the Balance Sheet	462,493
Non-agency mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	252,829
Market Value	258,944
Amount at which shown in the Balance Sheet	258,944
Commercial mortgage-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	409,682
Market Value	409,747
Amount at which shown in the Balance Sheet	409,747
Asset-backed
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	187,941
Market Value	188,358
Amount at which shown in the Balance Sheet	188,358
Fixed maturity investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	6,920,690
Market Value	6,891,244
Amount at which shown in the Balance Sheet	6,891,244
Short term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Market Value	1,368,379
Amount at which shown in the Balance Sheet	1,368,379
Equity investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Market Value	383,313
Amount at which shown in the Balance Sheet	383,313
Other investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Market Value	549,805
Amount at which shown in the Balance Sheet	549,805
Investments in other ventures, under equity method
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Market Value	124,227
Amount at which shown in the Balance Sheet	$ 124,227