Fair Values of Financial Instruments (Assets and Liabilities Measured on a Recurring Basis by Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Assets
Derivative instruments	$ 207,265	$ 1,355,989
Fair Value, Measurements, Recurring
Assets
Derivative instruments	207,265	1,355,989
Cash and cash equivalents	1,833,099	2,293,392
Assets	49,745,231	55,229,877
Liabilities
Fixed index annuities - embedded derivatives	8,451,482	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
Investments	39,248	161,765
Fair Value, Measurements, Recurring \| United States Government Sponsored Agencies
Assets
Investments	338,561	625,020
Fair Value, Measurements, Recurring \| United States Municipalities, States and Territories
Assets
Investments	3,765,163	4,527,671
Fair Value, Measurements, Recurring \| Foreign Government Obligations
Assets
Investments	198,396	205,096
Fair Value, Measurements, Recurring \| Corporate Securities
Assets
Investments	31,135,319	32,536,839
Fair Value, Measurements, Recurring \| Residential Mortgage Backed Securities
Assets
Investments	1,765,033	1,575,664
Fair Value, Measurements, Recurring \| Commercial Mortgage Backed Securities
Assets
Investments	5,296,112	5,786,279
Fair Value, Measurements, Recurring \| Other Asset Backed Securities
Assets
Investments	5,167,035	6,162,156
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1)
Assets
Derivative instruments	0	0
Cash and cash equivalents	1,833,099	2,293,392
Assets	1,866,070	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
Investments	32,963	155,945
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| United States Government Sponsored Agencies
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| United States Municipalities, States and Territories
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Foreign Government Obligations
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Corporate Securities
Assets
Investments	8	4
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Residential Mortgage Backed Securities
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Commercial Mortgage Backed Securities
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Other Asset Backed Securities
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Assets
Derivative instruments	207,265	1,355,989
Cash and cash equivalents	0	0
Assets	47,879,161	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
Investments	6,285	5,820
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| United States Government Sponsored Agencies
Assets
Investments	338,561	625,020
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| United States Municipalities, States and Territories
Assets
Investments	3,765,163	4,527,671
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Foreign Government Obligations
Assets
Investments	198,396	205,096
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate Securities
Assets
Investments	31,135,311	32,536,835
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Residential Mortgage Backed Securities
Assets
Investments	1,765,033	1,575,664
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial Mortgage Backed Securities
Assets
Investments	5,296,112	5,786,279
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Other Asset Backed Securities
Assets
Investments	5,167,035	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	8,451,482	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
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| United States Government Sponsored Agencies
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| United States Municipalities, States and Territories
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Foreign Government Obligations
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Corporate Securities
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Residential Mortgage Backed Securities
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Commercial Mortgage Backed Securities
Assets
Investments	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Other Asset Backed Securities
Assets
Investments	$ 0	$ 0

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Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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Fair value portion of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Also includes short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

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Fair value as of the balance sheet date of the embedded derivative or group of embedded derivatives classified as a liability.

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Fair value as of the balance sheet date of interest rate derivative assets, which includes all such derivative instruments in hedging and nonhedging relationships that are recognized as assets.

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