Fair Values of Financial Instruments (Fair Values and Carrying Amounts of Financial Instruments) (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Assets
Held for investment	$ 71,113	$ 76,460
Other investments	536,594	616,764
Coinsurance deposits	5,017,255	4,858,289
Liabilities
Policy benefit reserves	57,992,164	56,142,673
Interest Rate Caps
Assets
Derivative instruments	1,244	415
Carrying Amount
Assets
Available for sale securities	45,822,017	45,372,989
Held for investment	77,213	77,041
Mortgage loans on real estate	2,892,155	2,665,531
Derivative instruments	1,290,387	1,568,380
Other investments	536,594	616,764
Cash and cash equivalents	1,129,242	1,434,045
Coinsurance deposits	5,017,255	4,858,289
Counterparty collateral	179,512	186,108
Liabilities
Policy benefit reserves	57,633,261	55,786,011
Single premium immediate annuity (SPIA) benefit reserves	271,275	282,563
Notes payable	494,464	494,093
Subordinated debentures	242,875	242,565
Amounts due under repurchase agreements	116,399	0
Carrying Amount | Interest Rate Caps
Assets
Interest rate derivative assets	1,244	415
Carrying Amount | Interest Rate Swap
Assets
Interest rate derivative assets	1,125	0
Liabilities
Interest rate derivative liabilities	0	789
Fair Value
Assets
Available for sale securities	45,822,017	45,372,989
Held for investment	71,113	76,460
Mortgage loans on real estate	2,835,345	2,670,037
Derivative instruments	1,290,387	1,568,380
Other investments	528,867	605,894
Cash and cash equivalents	1,129,242	1,434,045
Coinsurance deposits	4,547,180	4,347,990
Counterparty collateral	179,512	186,108
Liabilities
Policy benefit reserves	48,792,935	46,344,931
Single premium immediate annuity (SPIA) benefit reserves	279,993	292,153
Notes payable	500,000	521,800
Subordinated debentures	238,703	244,117
Amounts due under repurchase agreements	116,399	0
Fair Value | Interest Rate Caps
Assets
Interest rate derivative assets	1,244	415
Fair Value | Interest Rate Swap
Assets
Interest rate derivative assets	1,125	0
Liabilities
Interest rate derivative liabilities	$ 0	$ 789