SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2017 USD ($)
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	$ 14,333,672
Amount shown on the Balance Sheet	14,784,210
U.S. government and agency
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,727,643
Fair Value	1,712,469
Amount shown on the Balance Sheet	1,712,469
Non-U.S. government
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	798,582
Fair Value	806,299
Amount shown on the Balance Sheet	806,299
Corporate debt
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	5,265,795
Fair Value	5,297,866
Amount shown on the Balance Sheet	5,297,866
Agency RMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,414,720
Fair Value	2,395,152
Amount shown on the Balance Sheet	2,395,152
CMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	776,715
Fair Value	777,728
Amount shown on the Balance Sheet	777,728
Non-Agency RMBS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	45,713
Fair Value	46,831
Amount shown on the Balance Sheet	46,831
ABS
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,432,884
Fair Value	1,436,281
Amount shown on the Balance Sheet	1,436,281
Municipals
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	149,167
Fair Value	149,380
Amount shown on the Balance Sheet	149,380
Fixed maturities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	12,611,219
Fair Value	12,622,006
Amount shown on the Balance Sheet	12,622,006
Mortgage loans
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	325,062
Amount shown on the Balance Sheet	325,062
Short-term investments
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	83,661
Amount shown on the Balance Sheet	83,661
Equity securities
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	635,511
Amount shown on the Balance Sheet	635,511
Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	667,432
Amount shown on the Balance Sheet	1,009,373
Equity Method Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	0
Amount shown on the Balance Sheet	$ 108,597