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]
Fair Value	$ 13,048,072
Amount shown on the Balance Sheet	13,459,507
US Government and Agency [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,681,425
Fair Value	1,656,069
Amount shown on the Balance Sheet	1,656,069
Non-US Government [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	613,282
Fair Value	565,834
Amount shown on the Balance Sheet	565,834
Corporate Debt [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	4,633,834
Fair Value	4,600,743
Amount shown on the Balance Sheet	4,600,743
Agency RMBS [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	2,487,837
Fair Value	2,465,135
Amount shown on the Balance Sheet	2,465,135
CMBS [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	664,368
Fair Value	666,237
Amount shown on the Balance Sheet	666,237
Non-Agency RMBS [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	57,316
Fair Value	56,921
Amount shown on the Balance Sheet	56,921
ABS [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	1,221,813
Fair Value	1,222,214
Amount shown on the Balance Sheet	1,222,214
Municipals [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	163,441
Fair Value	163,961
Amount shown on the Balance Sheet	163,961
Fixed Maturities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Amortized Cost	11,523,316
Fair Value	11,397,114
Amount shown on the Balance Sheet	11,397,114
Mortgage loans [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	349,969
Amount shown on the Balance Sheet	349,969
Short-term Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	127,461
Amount shown on the Balance Sheet	127,461
Equity Securities [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	638,744
Amount shown on the Balance Sheet	638,744
Other Investments [Member]
Summary of Investments, Other than Investments in Related Parties, Reportable Data [Line Items]
Fair Value	534,784
Amount shown on the Balance Sheet	830,219
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	$ 116,000