Investments - Summary of Fixed Maturities Available for Sale by Component (Details) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 13,944,467	$ 13,251,871
Debt Securities Fair Value	15,837,700	$ 13,758,024
Fixed maturities available for sale [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	13,944,467
Gross Unrealized Gains	2,013,592
Gross Unrealized Losses	(120,359)
Debt Securities Fair Value	$ 15,837,700
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	$ 382,603
Gross Unrealized Gains	33,946
Gross Unrealized Losses	(431)
Debt Securities Fair Value	$ 416,118
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	$ 1,272,731
Gross Unrealized Gains	178,622
Gross Unrealized Losses	(188)
Debt Securities Fair Value	$ 1,451,165
Percentage of Total Fixed Maturities At Fair Value	9.00%
Fixed maturities available for sale [Member] | Foreign governments [Member]
Schedule of Available-for-sale Securities [Line Items]
Fixed Maturities, Available for Sale, Amortized Cost	$ 22,732
Gross Unrealized Gains	2,835
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 25,567
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	$ 11,748,152
Gross Unrealized Gains	1,721,900
Gross Unrealized Losses	(102,478)
Debt Securities Fair Value	$ 13,367,574
Percentage of Total Fixed Maturities At Fair Value	85.00%
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	$ 2,871,873
Gross Unrealized Gains	415,796
Gross Unrealized Losses	(24,624)
Debt Securities Fair Value	$ 3,263,045
Percentage of Total Fixed Maturities At Fair Value	21.00%
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	$ 1,952,885
Gross Unrealized Gains	395,477
Gross Unrealized Losses	(3,816)
Debt Securities Fair Value	$ 2,344,546
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	$ 1,565,677
Gross Unrealized Gains	148,044
Gross Unrealized Losses	(57,686)
Debt Securities Fair Value	$ 1,656,035
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	$ 5,357,717
Gross Unrealized Gains	762,583
Gross Unrealized Losses	(16,352)
Debt Securities Fair Value	$ 6,103,948
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	$ 60,857
Gross Unrealized Gains	14,062
Gross Unrealized Losses	(11,637)
Debt Securities Fair Value	$ 63,282
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	$ 56,751
Gross Unrealized Gains	2,390
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 59,141
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	$ 400,641
Gross Unrealized Gains	59,837
Gross Unrealized Losses	(5,625)
Debt Securities Fair Value	$ 454,853
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	$ 372,030
Gross Unrealized Gains	58,148
Gross Unrealized Losses	(5,625)
Debt Securities Fair Value	$ 424,553
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	$ 28,611
Gross Unrealized Gains	1,689
Gross Unrealized Losses	0
Debt Securities Fair Value	$ 30,300
Percentage of Total Fixed Maturities At Fair Value	0.00%