Fair Values of Financial Instruments (Assets and Liabilities Measured on a Recurring Basis by Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Assets
Derivative instruments	$ 1,310,954	$ 1,355,989
Fair Value, Measurements, Recurring
Assets
Derivative instruments	1,310,954	1,355,989
Cash and cash equivalents	9,095,522	2,293,392
Assets	57,945,369	55,229,877
Liabilities
Fixed index annuities - embedded derivatives	7,938,281	9,624,395
Fair Value, Measurements, Recurring | Interest Rate Caps
Assets
Interest rate derivative assets		6
Fair Value, Measurements, Recurring | United States Government Full Faith and Credit
Assets
Available for sale securities	39,771	161,765
Fair Value, Measurements, Recurring | United States Government Sponsored Agencies
Assets
Available for sale securities	1,039,551	625,020
Fair Value, Measurements, Recurring | United States Municipalities, States and Territories
Assets
Available for sale securities	3,776,131	4,527,671
Fair Value, Measurements, Recurring | Foreign Government Obligations
Assets
Available for sale securities	202,706	205,096
Fair Value, Measurements, Recurring | Corporate Securities
Assets
Available for sale securities	31,156,827	32,536,839
Fair Value, Measurements, Recurring | Residential Mortgage Backed Securities
Assets
Available for sale securities	1,512,831	1,575,664
Fair Value, Measurements, Recurring | Commercial Mortgage Backed Securities
Assets
Available for sale securities	4,261,227	5,786,279
Fair Value, Measurements, Recurring | Other Asset Backed Securities
Assets
Available for sale securities	5,549,849	6,162,156
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Assets
Derivative instruments	0	0
Cash and cash equivalents	9,095,522	2,293,392
Assets	9,129,470	2,449,341
Liabilities
Fixed index annuities - embedded derivatives	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Interest Rate Caps
Assets
Interest rate derivative assets		0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | United States Government Full Faith and Credit
Assets
Available for sale securities	33,940	155,945
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | United States Government Sponsored Agencies
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | United States Municipalities, States and Territories
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Foreign Government Obligations
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Corporate Securities
Assets
Available for sale securities	8	4
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Residential Mortgage Backed Securities
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Commercial Mortgage Backed Securities
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1) | Other Asset Backed Securities
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets
Derivative instruments	1,310,954	1,355,989
Cash and cash equivalents	0	0
Assets	48,815,899	52,780,536
Liabilities
Fixed index annuities - embedded derivatives	0	0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Interest Rate Caps
Assets
Interest rate derivative assets		6
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | United States Government Full Faith and Credit
Assets
Available for sale securities	5,831	5,820
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | United States Government Sponsored Agencies
Assets
Available for sale securities	1,039,551	625,020
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | United States Municipalities, States and Territories
Assets
Available for sale securities	3,776,131	4,527,671
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Foreign Government Obligations
Assets
Available for sale securities	202,706	205,096
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Corporate Securities
Assets
Available for sale securities	31,156,819	32,536,835
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Residential Mortgage Backed Securities
Assets
Available for sale securities	1,512,831	1,575,664
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Commercial Mortgage Backed Securities
Assets
Available for sale securities	4,261,227	5,786,279
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2) | Other Asset Backed Securities
Assets
Available for sale securities	5,549,849	6,162,156
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets
Derivative instruments	0	0
Cash and cash equivalents	0	0
Assets	0	0
Liabilities
Fixed index annuities - embedded derivatives	7,938,281	9,624,395
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Interest Rate Caps
Assets
Interest rate derivative assets		0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | United States Government Full Faith and Credit
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | United States Government Sponsored Agencies
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | United States Municipalities, States and Territories
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Foreign Government Obligations
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Corporate Securities
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Residential Mortgage Backed Securities
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Commercial Mortgage Backed Securities
Assets
Available for sale securities	0	0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3) | Other Asset Backed Securities
Assets
Available for sale securities	$ 0	$ 0