Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 13,778,254	$ 13,251,871
Debt Securities Fair Value	15,440,090	$ 13,758,024
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	13,778,254
Gross Unrealized Gains	1,873,093
Gross Unrealized Losses	(211,257)
Debt Securities Fair Value	$ 15,440,090
Percentage of Total Fixed Maturities At Fair Value	100.00%
Fixed maturities available for sale [Member] | U.S. Government direct, guaranteed, and government-sponsored enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 380,528
Gross Unrealized Gains	33,040
Gross Unrealized Losses	(432)
Debt Securities Fair Value	$ 413,136
Percentage of Total Fixed Maturities At Fair Value	3.00%
Fixed maturities available for sale [Member] | States, municipalities, and political subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 1,277,303
Gross Unrealized Gains	184,320
Gross Unrealized Losses	(199)
Debt Securities Fair Value	$ 1,461,424
Percentage of Total Fixed Maturities At Fair Value	10.00%
Fixed maturities available for sale [Member] | Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 22,468
Gross Unrealized Gains	2,773
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 25,241
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] | Corporates [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 11,568,049
Gross Unrealized Gains	1,582,460
Gross Unrealized Losses	(194,144)
Debt Securities Fair Value	$ 12,956,365
Percentage of Total Fixed Maturities At Fair Value	84.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 2,800,108
Gross Unrealized Gains	388,055
Gross Unrealized Losses	(48,336)
Debt Securities Fair Value	$ 3,139,827
Percentage of Total Fixed Maturities At Fair Value	20.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 1,950,172
Gross Unrealized Gains	377,501
Gross Unrealized Losses	(7,370)
Debt Securities Fair Value	$ 2,320,303
Percentage of Total Fixed Maturities At Fair Value	15.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Energy [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 1,566,871
Gross Unrealized Gains	114,956
Gross Unrealized Losses	(83,315)
Debt Securities Fair Value	$ 1,598,512
Percentage of Total Fixed Maturities At Fair Value	10.00%
Fixed maturities available for sale [Member] | Corporates [Member] | Other corporate sectors [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 5,250,898
Gross Unrealized Gains	701,948
Gross Unrealized Losses	(55,123)
Debt Securities Fair Value	$ 5,897,723
Percentage of Total Fixed Maturities At Fair Value	39.00%
Fixed maturities available for sale [Member] | Collateralized debt obligations [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 62,174
Gross Unrealized Gains	13,617
Gross Unrealized Losses	(11,728)
Debt Securities Fair Value	$ 64,063
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] | Other asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 56,914
Gross Unrealized Gains	1,989
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 58,903
Percentage of Total Fixed Maturities At Fair Value	0.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 410,818
Gross Unrealized Gains	54,894
Gross Unrealized Losses	(4,754)
Debt Securities Fair Value	$ 460,958
Percentage of Total Fixed Maturities At Fair Value	3.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Financial [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 382,195
Gross Unrealized Gains	52,900
Gross Unrealized Losses	(4,754)
Debt Securities Fair Value	$ 430,341
Percentage of Total Fixed Maturities At Fair Value	3.00%
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member] | Utilities [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 28,623
Gross Unrealized Gains	1,994
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 30,617
Percentage of Total Fixed Maturities At Fair Value	0.00%