SCHEDULE I - SUMMARY OF INVESTMENTS - OTHER THAN INVESTMENTS IN RELATED PARTIES (Details) $ in Thousands	Dec. 31, 2018 USD ($)
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value	$ 12,705,462
Amount shown on the balance sheet	13,155,560
Fixed maturities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	11,616,312
Fair value	11,435,347
Amount shown on the balance sheet	11,435,347
U.S. government and agency
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	1,520,142
Fair value	1,515,697
Amount shown on the balance sheet	1,515,697
Non-U.S. government
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	507,550
Fair value	493,016
Amount shown on the balance sheet	493,016
Corporate debt
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	4,990,279
Fair value	4,876,921
Amount shown on the balance sheet	4,876,921
Agency RMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	1,666,684
Fair value	1,643,308
Amount shown on the balance sheet	1,643,308
CMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	1,103,507
Fair value	1,092,530
Amount shown on the balance sheet	1,092,530
Non-Agency RMBS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	40,732
Fair value	40,687
Amount shown on the balance sheet	40,687
ABS
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	1,651,350
Fair value	1,637,603
Amount shown on the balance sheet	1,637,603
Municipals
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Amortized cost	136,068
Fair value	135,585
Amount shown on the balance sheet	135,585
Mortgage loans
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value	298,650
Amount shown on the balance sheet	298,650
Short-term investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value	144,040
Amount shown on the balance sheet	144,040
Equity securities
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value	381,633
Amount shown on the balance sheet	381,633
Other Investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value	445,792
Amount shown on the balance sheet	787,787
Equity method investments
SEC Schedule, 12-15, Insurance Companies, Summary of Investments, Other than Investments in Related Parties [Line Items]
Fair value	0
Amount shown on the balance sheet	$ 108,103