Investments - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	$ 15,874,131	$ 15,245,861
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	95.80%
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	4.20%
Fixed maturities available for sale [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	$ 15,874,131
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	381,161
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	1,303,099
Fixed maturities available for sale [Member] | Foreign governments [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	21,524
Fixed maturities available for sale [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	13,552,342
Fixed maturities available for sale [Member] | Corporates [Member] | Financial [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	3,434,939
Fixed maturities available for sale [Member] | Corporates [Member] | Utilities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	2,154,670
Fixed maturities available for sale [Member] | Corporates [Member] | Energy [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	1,703,972
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	6,258,761
Fixed maturities available for sale [Member] | Collateralized debt obligations [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	67,843
Fixed maturities available for sale [Member] | Other asset-backed securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	105,187
Fixed maturities available for sale [Member] | Redeemable preferred stocks [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	442,975
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	413,005
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	29,970
Fixed maturities available for sale [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	4
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	4
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	0
Fixed maturities available for sale [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	15,206,296
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	381,157
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	1,303,099
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	21,524
Fixed maturities available for sale [Member] | Level 2 [Member] | Corporates [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	12,966,354
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	3,372,832
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	2,000,220
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	1,663,113
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	5,930,189
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	91,187
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	442,975
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	413,005
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	29,970
Fixed maturities available for sale [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	667,831
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	585,988
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	62,107
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	154,450
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	40,859
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	328,572
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	67,843
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	14,000
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,602,250; 2016—$14,188,050)	$ 0