Schedule I. Summary of Investments Other Than Investments in Related Parties (Details) $ in Thousands	Dec. 31, 2019 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	$ 17,368,789
Amount at which shown in the Balance Sheet	17,368,789
U.S. treasuries
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	4,439,533
Fair Value	4,467,345
Amount at which shown in the Balance Sheet	4,467,345
Agencies
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	342,162
Fair Value	343,031
Amount at which shown in the Balance Sheet	343,031
Non-U.S. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	495,465
Fair Value	497,392
Amount at which shown in the Balance Sheet	497,392
Non-U.S. government-backed corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	321,303
Fair Value	321,356
Amount at which shown in the Balance Sheet	321,356
Corporate
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	3,010,615
Fair Value	3,075,660
Amount at which shown in the Balance Sheet	3,075,660
Agency mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	1,130,746
Fair Value	1,148,499
Amount at which shown in the Balance Sheet	1,148,499
Non-agency mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	282,267
Fair Value	294,604
Amount at which shown in the Balance Sheet	294,604
Commercial mortgage-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	489,352
Fair Value	468,698
Amount at which shown in the Balance Sheet	468,698
Asset-backed
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	555,971
Fair Value	555,070
Amount at which shown in the Balance Sheet	555,070
Fixed maturity investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized Cost or Cost	11,067,414
Fair Value	11,171,655
Amount at which shown in the Balance Sheet	11,171,655
Short term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	4,566,277
Amount at which shown in the Balance Sheet	4,566,277
Equity investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	436,931
Amount at which shown in the Balance Sheet	436,931
Other investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	1,087,377
Amount at which shown in the Balance Sheet	1,087,377
Investments in other ventures, under equity method
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair Value	106,549
Amount at which shown in the Balance Sheet	$ 106,549