Significant Accounting Policies (Annuity Deposits (Net of Coinsurance), By Product Type) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) NMOs	Dec. 31, 2016 USD ($) NMOs	Dec. 31, 2015 USD ($) NMOs
Product Information [Line Items]
Fixed index annuities	$ 3,668,121	$ 5,035,818	$ 6,491,981
Annual reset fixed rate annuities	74,572	63,582	44,715
Multi-year fixed rate annuities	22,291	256,894	42,709
Single premium immediate annuities (SPIA)	24,946	35,851	32,752
Annuity deposits, net of coinsurance	$ 3,789,930	$ 5,392,145	$ 6,612,157
National marketing organizations, number of organizations accounting for more than 10% of annuity deposits collected | NMOs	2	1	1
National Marketing Organization, Greater than 10%, NMO 1 [Member]
Product Information [Line Items]
National marketing organizations, percent of annuity deposits collected individually	14.00%	15.00%	22.00%
National Marketing Organization, Greater than 10%, NMO 2 [Member]
Product Information [Line Items]
National marketing organizations, percent of annuity deposits collected individually	10.00%