v3.8.0.1
Investments - Schedule of Investments Reconciliation (Details) - USD ($)
$ in Thousands
 6 Months Ended 12 Months Ended
Jun. 30, 2017
Dec. 31, 2016
Sep. 30, 2017
Schedule of Investments [Abstract]      
Fair value of securities transferred from AFS to HTM $ 391,934 $ 155,538  
AFS transferred to HTM securities, unrealized gain $ 4,313 15,770  
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   4,005,642 $ 3,844,996
Fixed Maturities, available-for-sale, Gross unrealized gains   50,759 71,850
Fixed Maturities, available-for-sale, Gross unrealized losses   (84,735) (32,259)
Fixed Maturities, available-for-sale, Fair value   3,971,666 3,884,587
Fixed Maturities, held-to-maturity, Original or amortized cost   752,212 1,118,368
Fixed Maturities, held-to-maturity, Gross unrealized gains   16,370 34,486
Fixed Maturities, held-to-maturity, Gross unrealized losses   (2,447) (748)
Fixed Maturities, held-to-maturity, Fair value   766,135 1,152,106
Other Investments, Original or amortized cost   10,057 5,640
Other Investments, Gross unrealized gains   3,003 1,401
Other Investments, Gross unrealized losses   0 0
Other investments   13,060 7,041
Total investments, Original or amortized cost   4,767,911 4,969,004
Investments, Gross unrealized gains   70,132 107,737
Investments, Gross unrealized losses   (87,182) (33,007)
Total investments   4,750,861 5,043,734
U.S. treasury bonds [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   5,186 5,194
Fixed Maturities, available-for-sale, Gross unrealized gains   238 150
Fixed Maturities, available-for-sale, Gross unrealized losses   (11) (8)
Fixed Maturities, available-for-sale, Fair value   5,413 5,336
U.S. agency bonds – mortgage-backed [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   1,720,436 2,004,645
Fixed Maturities, available-for-sale, Gross unrealized gains   12,867 14,208
Fixed Maturities, available-for-sale, Gross unrealized losses   (17,265) (12,451)
Fixed Maturities, available-for-sale, Fair value   1,716,038 2,006,402
U.S. agency bonds – other [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   18,082  
Fixed Maturities, available-for-sale, Gross unrealized gains   20  
Fixed Maturities, available-for-sale, Gross unrealized losses   0  
Fixed Maturities, available-for-sale, Fair value   18,102  
Non-U.S. government and supranational bonds [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   35,158 33,392
Fixed Maturities, available-for-sale, Gross unrealized gains   73 216
Fixed Maturities, available-for-sale, Gross unrealized losses   (5,297) (2,012)
Fixed Maturities, available-for-sale, Fair value   29,934 31,596
Asset-backed securities [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   217,232 257,969
Fixed Maturities, available-for-sale, Gross unrealized gains   3,713 4,456
Fixed Maturities, available-for-sale, Gross unrealized losses   (69) (164)
Fixed Maturities, available-for-sale, Fair value   220,876 262,261
Corporate bonds [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   1,947,347 1,541,296
Fixed Maturities, available-for-sale, Gross unrealized gains   30,951 52,717
Fixed Maturities, available-for-sale, Gross unrealized losses   (62,093) (17,624)
Fixed Maturities, available-for-sale, Fair value   1,916,205 1,576,389
Fixed Maturities, held-to-maturity, Original or amortized cost   752,212 1,057,943
Fixed Maturities, held-to-maturity, Gross unrealized gains   16,370 34,027
Fixed Maturities, held-to-maturity, Gross unrealized losses   (2,447) (748)
Fixed Maturities, held-to-maturity, Fair value   766,135 1,091,222
Municipal bonds [Member]      
Schedule of Investments [Line Items]      
Fixed Maturities, available-for-sale, Original or amortized cost   62,201 2,500
Fixed Maturities, available-for-sale, Gross unrealized gains   2,897 103
Fixed Maturities, available-for-sale, Gross unrealized losses   0 0
Fixed Maturities, available-for-sale, Fair value   $ 65,098 2,603
Fixed Maturities, held-to-maturity, Original or amortized cost     60,425
Fixed Maturities, held-to-maturity, Gross unrealized gains     459
Fixed Maturities, held-to-maturity, Gross unrealized losses     0
Fixed Maturities, held-to-maturity, Fair value     $ 60,884