Investments - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 15,837,700	$ 13,758,024
Percent of total	100.00%
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	0.00%
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	95.90%
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	4.10%
Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 15,837,700
Fixed maturities available for sale [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	416,118
Fixed maturities available for sale [Member] | States, municipalities, and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,451,165
Fixed maturities available for sale [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	25,567
Fixed maturities available for sale [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	13,367,574
Fixed maturities available for sale [Member] | Corporates [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	3,263,045
Fixed maturities available for sale [Member] | Corporates [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,344,546
Fixed maturities available for sale [Member] | Corporates [Member] | Energy [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,656,035
Fixed maturities available for sale [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	6,103,948
Fixed maturities available for sale [Member] | Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	63,282
Fixed maturities available for sale [Member] | Other asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	59,141
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	454,853
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	424,553
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	30,300
Fixed maturities available for sale [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	17
Fixed maturities available for sale [Member] | Level 1 [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	17
Fixed maturities available for sale [Member] | Level 1 [Member] | States, municipalities, and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Corporates [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Corporates [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Corporates [Member] | Energy [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Other asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Redeemable preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 1 [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	15,186,648
Fixed maturities available for sale [Member] | Level 2 [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	416,101
Fixed maturities available for sale [Member] | Level 2 [Member] | States, municipalities, and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,451,165
Fixed maturities available for sale [Member] | Level 2 [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	25,567
Fixed maturities available for sale [Member] | Level 2 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	12,779,821
Fixed maturities available for sale [Member] | Level 2 [Member] | Corporates [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	3,199,394
Fixed maturities available for sale [Member] | Level 2 [Member] | Corporates [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	2,182,029
Fixed maturities available for sale [Member] | Level 2 [Member] | Corporates [Member] | Energy [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	1,627,142
Fixed maturities available for sale [Member] | Level 2 [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	5,771,256
Fixed maturities available for sale [Member] | Level 2 [Member] | Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 2 [Member] | Other asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	59,141
Fixed maturities available for sale [Member] | Level 2 [Member] | Redeemable preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	454,853
Fixed maturities available for sale [Member] | Level 2 [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	424,553
Fixed maturities available for sale [Member] | Level 2 [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	30,300
Fixed maturities available for sale [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	651,035
Fixed maturities available for sale [Member] | Level 3 [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 3 [Member] | States, municipalities, and political subdivisions [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 3 [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 3 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	587,753
Fixed maturities available for sale [Member] | Level 3 [Member] | Corporates [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	63,651
Fixed maturities available for sale [Member] | Level 3 [Member] | Corporates [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	162,517
Fixed maturities available for sale [Member] | Level 3 [Member] | Corporates [Member] | Energy [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	28,893
Fixed maturities available for sale [Member] | Level 3 [Member] | Corporates [Member] | Other corporate sectors [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	332,692
Fixed maturities available for sale [Member] | Level 3 [Member] | Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	63,282
Fixed maturities available for sale [Member] | Level 3 [Member] | Other asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 3 [Member] | Redeemable preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 3 [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	0
Fixed maturities available for sale [Member] | Level 3 [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fair value fixed maturities measured on recurring basis	$ 0