Fair Values of Financial Instruments (Assets and Liabilities Measured on a Recurring Basis by Fair Value Hierarchy) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Assets
Derivative instruments	$ 963,081	$ 205,149
Fair Value, Measurements, Recurring
Assets
Derivative instruments	963,081	205,149
Cash and cash equivalents	1,138,737	344,396
Counterparty collateral		33,101
Assets	54,265,532	46,514,761
Liabilities
Fixed index annuities - embedded derivatives	9,381,094	8,165,405
Liabilities	9,382,000
Fair Value, Measurements, Recurring \| Interest Rate Caps
Assets
Interest rate derivative assets	17	597
Fair Value, Measurements, Recurring \| Interest Rate Swap
Assets
Interest rate derivative assets		354
Liabilities
Interest rate derivative liabilities	906
Fair Value, Measurements, Recurring \| United States Government Full Faith and Credit
Assets
Investments	12,343	11,652
Fair Value, Measurements, Recurring \| United States Government Sponsored Agencies
Assets
Investments	695,030	1,138,529
Fair Value, Measurements, Recurring \| United States Municipalities, States and Territories
Assets
Investments	4,517,661	4,126,267
Fair Value, Measurements, Recurring \| Foreign Government Obligations
Assets
Investments	207,420	230,274
Fair Value, Measurements, Recurring \| Corporate Securities
Assets
Investments	32,897,127	28,371,514
Fair Value, Measurements, Recurring \| Residential Mortgage Backed Securities
Assets
Investments	1,557,156	1,202,159
Fair Value, Measurements, Recurring \| Commercial Mortgage Backed Securities
Assets
Investments	5,839,510	5,379,003
Fair Value, Measurements, Recurring \| Other Asset Backed Securities
Assets
Investments	6,128,137	5,464,329
Fair Value, Measurements, Recurring \| Equity Securities
Assets
Investments	309,313	7,437
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1)
Assets
Derivative instruments	0	0
Cash and cash equivalents	1,138,737	344,396
Counterparty collateral		0
Assets	1,447,043	350,303
Liabilities
Fixed index annuities - embedded derivatives	0	0
Liabilities	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Interest Rate Caps
Assets
Interest rate derivative assets	0	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| Interest Rate Swap
Assets
Interest rate derivative assets		0
Liabilities
Interest rate derivative liabilities	0
Fair Value, Measurements, Recurring \| Quoted Prices in Active Markets (Level 1) \| United States Government Full Faith and Credit
Assets
Investments	6,421	5,900
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	15	7
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 \| Quoted Prices in Active Markets (Level 1) \| Equity Securities
Assets
Investments	301,870	0
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Assets
Derivative instruments	963,081	205,149
Cash and cash equivalents	0	0
Counterparty collateral		33,101
Assets	52,818,489	46,164,458
Liabilities
Fixed index annuities - embedded derivatives	0	0
Liabilities	906
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Interest Rate Caps
Assets
Interest rate derivative assets	17	597
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Interest Rate Swap
Assets
Interest rate derivative assets		354
Liabilities
Interest rate derivative liabilities	906
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| United States Government Full Faith and Credit
Assets
Investments	5,922	5,752
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| United States Government Sponsored Agencies
Assets
Investments	695,030	1,138,529
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| United States Municipalities, States and Territories
Assets
Investments	4,517,661	4,126,267
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Foreign Government Obligations
Assets
Investments	207,420	230,274
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Corporate Securities
Assets
Investments	32,897,112	28,371,507
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Residential Mortgage Backed Securities
Assets
Investments	1,557,156	1,202,159
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Commercial Mortgage Backed Securities
Assets
Investments	5,839,510	5,379,003
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Other Asset Backed Securities
Assets
Investments	6,128,137	5,464,329
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Equity Securities
Assets
Investments	7,443	7,437
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Assets
Derivative instruments	0	0
Cash and cash equivalents	0	0
Counterparty collateral		0
Assets	0	0
Liabilities
Fixed index annuities - embedded derivatives	9,381,094	8,165,405
Liabilities	9,381,094
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Interest Rate Caps
Assets
Interest rate derivative assets	0	0
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Interest Rate Swap
Assets
Interest rate derivative assets		0
Liabilities
Interest rate derivative liabilities	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
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Equity Securities
Assets
Investments	$ 0	$ 0

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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 portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

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Fair value of financial and nonfinancial obligations.

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