Investments - Assets Measured at Fair Value on Recurring Basis (Details) - USD ($) $ in Thousands	Sep. 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—$14,914,580; 2016—$14,188,050)	$ 16,652,913	$ 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	0.30%
Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	95.70%
Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Percent of total	4.00%
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—$14,914,580; 2016—$14,188,050)	$ 16,652,913
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—$14,914,580; 2016—$14,188,050)	399,687
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—$14,914,580; 2016—$14,188,050)	1,288,104
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—$14,914,580; 2016—$14,188,050)	22,546
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—$14,914,580; 2016—$14,188,050)	14,297,850
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—$14,914,580; 2016—$14,188,050)	3,599,934
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—$14,914,580; 2016—$14,188,050)	2,269,924
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—$14,914,580; 2016—$14,188,050)	1,759,280
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—$14,914,580; 2016—$14,188,050)	6,668,712
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—$14,914,580; 2016—$14,188,050)	69,768
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—$14,914,580; 2016—$14,188,050)	150,225
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—$14,914,580; 2016—$14,188,050)	424,733
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—$14,914,580; 2016—$14,188,050)	393,951
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—$14,914,580; 2016—$14,188,050)	30,782
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—$14,914,580; 2016—$14,188,050)	42,325
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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	39,100
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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	3,225
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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	3,225
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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	15,941,954
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—$14,914,580; 2016—$14,188,050)	399,687
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—$14,914,580; 2016—$14,188,050)	1,249,004
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—$14,914,580; 2016—$14,188,050)	22,546
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—$14,914,580; 2016—$14,188,050)	13,710,142
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—$14,914,580; 2016—$14,188,050)	3,537,728
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—$14,914,580; 2016—$14,188,050)	2,114,600
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—$14,914,580; 2016—$14,188,050)	1,717,538
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—$14,914,580; 2016—$14,188,050)	6,340,276
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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	135,842
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—$14,914,580; 2016—$14,188,050)	424,733
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—$14,914,580; 2016—$14,188,050)	393,951
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—$14,914,580; 2016—$14,188,050)	30,782
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—$14,914,580; 2016—$14,188,050)	668,634
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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	584,483
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—$14,914,580; 2016—$14,188,050)	62,206
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—$14,914,580; 2016—$14,188,050)	155,324
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—$14,914,580; 2016—$14,188,050)	41,742
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—$14,914,580; 2016—$14,188,050)	325,211
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—$14,914,580; 2016—$14,188,050)	69,768
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—$14,914,580; 2016—$14,188,050)	14,383
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—$14,914,580; 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—$14,914,580; 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—$14,914,580; 2016—$14,188,050)	$ 0