Investments - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Jun. 30, 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—$14,651,551; 2016—$14,188,050)	$ 16,318,286	$ 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	95.90%
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]
Fixed maturities—available for sale, at fair value (amortized cost: 2017—$14,651,551; 2016—$14,188,050)	$ 16,318,286
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—$14,651,551; 2016—$14,188,050)	394,187
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—$14,651,551; 2016—$14,188,050)	1,310,698
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—$14,651,551; 2016—$14,188,050)	22,312
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—$14,651,551; 2016—$14,188,050)	13,964,289
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—$14,651,551; 2016—$14,188,050)	3,539,992
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—$14,651,551; 2016—$14,188,050)	2,220,721
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—$14,651,551; 2016—$14,188,050)	1,724,383
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—$14,651,551; 2016—$14,188,050)	6,479,193
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—$14,651,551; 2016—$14,188,050)	66,245
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—$14,651,551; 2016—$14,188,050)	129,611
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—$14,651,551; 2016—$14,188,050)	430,944
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—$14,651,551; 2016—$14,188,050)	400,862
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—$14,651,551; 2016—$14,188,050)	30,082
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—$14,651,551; 2016—$14,188,050)	0
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—$14,651,551; 2016—$14,188,050)	0
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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 2016—$14,188,050)	15,651,602
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—$14,651,551; 2016—$14,188,050)	394,187
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—$14,651,551; 2016—$14,188,050)	1,310,698
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—$14,651,551; 2016—$14,188,050)	22,312
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—$14,651,551; 2016—$14,188,050)	13,378,003
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—$14,651,551; 2016—$14,188,050)	3,477,619
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—$14,651,551; 2016—$14,188,050)	2,065,431
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—$14,651,551; 2016—$14,188,050)	1,683,064
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—$14,651,551; 2016—$14,188,050)	6,151,889
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—$14,651,551; 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—$14,651,551; 2016—$14,188,050)	115,458
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—$14,651,551; 2016—$14,188,050)	430,944
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—$14,651,551; 2016—$14,188,050)	400,862
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—$14,651,551; 2016—$14,188,050)	30,082
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—$14,651,551; 2016—$14,188,050)	666,684
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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 2016—$14,188,050)	586,286
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—$14,651,551; 2016—$14,188,050)	62,373
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—$14,651,551; 2016—$14,188,050)	155,290
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—$14,651,551; 2016—$14,188,050)	41,319
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—$14,651,551; 2016—$14,188,050)	327,304
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—$14,651,551; 2016—$14,188,050)	66,245
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—$14,651,551; 2016—$14,188,050)	14,153
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—$14,651,551; 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—$14,651,551; 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—$14,651,551; 2016—$14,188,050)	$ 0