Investments - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 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	$ 15,440,090	$ 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	0.10%
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	95.80%
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	$ 15,440,090
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	413,136
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	1,461,424
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	25,241
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	12,956,365
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	3,139,827
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	2,320,303
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	1,598,512
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	5,897,723
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	64,063
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	58,903
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	460,958
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	430,341
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	30,617
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	22,194
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	18
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	12,008
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	12,008
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	10,168
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	10,168
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	14,784,668
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	413,118
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	1,461,424
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	25,241
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	12,375,192
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	3,076,036
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	2,168,991
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	1,569,212
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	5,560,953
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	58,903
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	450,790
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	420,173
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	30,617
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	633,228
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	569,165
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	63,791
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	139,304
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	29,300
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	336,770
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	64,063
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