v3.8.0.1
Investments - Credit Ratings (Details) - USD ($)
 3 Months Ended 9 Months Ended
Sep. 30, 2017
Sep. 30, 2016
Sep. 30, 2017
Sep. 30, 2016
Dec. 31, 2016
Schedule of Fixed Maturities Table [Line Items]          
OTTI $ 0 $ 0 $ 0 $ 0  
Fixed Maturities, Amortized cost 4,963,364,000   4,963,364,000   $ 4,757,854,000
Fixed Maturities, Fair value $ 5,036,693,000   $ 5,036,693,000   $ 4,737,801,000
Fixed maturities, % of Total fair value 100.00%   100.00%   100.00%
AAA [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 169,514,000   $ 169,514,000   $ 170,515,000
Fixed Maturities, Fair value $ 173,589,000   $ 173,589,000   $ 171,090,000
Fixed maturities, % of Total fair value 3.40%   3.40%   3.60%
AA Plus, AA, AA Minus [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 238,268,000   $ 238,268,000   $ 238,315,000
Fixed Maturities, Fair value $ 244,507,000   $ 244,507,000   $ 237,169,000
Fixed maturities, % of Total fair value 4.90%   4.90%   5.00%
A Plus, A, A Minus [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 1,395,001,000   $ 1,395,001,000   $ 1,386,023,000
Fixed Maturities, Fair value $ 1,421,504,000   $ 1,421,504,000   $ 1,374,860,000
Fixed maturities, % of Total fair value 28.30%   28.30%   29.00%
BBB Plus, BBB, BBB Minus [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 1,093,491,000   $ 1,093,491,000   $ 1,053,529,000
Fixed Maturities, Fair value $ 1,125,119,000   $ 1,125,119,000   $ 1,047,376,000
Fixed maturities, % of Total fair value 22.30%   22.30%   22.20%
BB Plus or Lower [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 57,251,000   $ 57,251,000   $ 165,768,000
Fixed Maturities, Fair value $ 60,236,000   $ 60,236,000   $ 167,753,000
Fixed maturities, % of Total fair value 1.20%   1.20%   3.50%
U.S. treasury bonds [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 5,194,000   $ 5,194,000   $ 5,186,000
Fixed Maturities, Fair value $ 5,336,000   $ 5,336,000   $ 5,413,000
Fixed maturities, % of Total fair value 0.10%   0.10%   0.10%
U.S. agency bonds [Member]          
Schedule of Fixed Maturities Table [Line Items]          
Fixed Maturities, Amortized cost $ 2,004,645,000   $ 2,004,645,000   $ 1,738,518,000
Fixed Maturities, Fair value $ 2,006,402,000   $ 2,006,402,000   $ 1,734,140,000
Fixed maturities, % of Total fair value 39.80%   39.80%   36.60%