Certain Long-Duration Contracts with Guarantees (Liabilities for Guarantee Benefits) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	$ 9,514,220	$ 8,858,768	$ 8,276,441
Incurred guarantee benefits	1,228,755	628,842	839,578
Paid guarantee benefits	(41,961)	(76,447)	(57,770)
Changes in unrealized investment gains and losses	156,348	103,057	(199,481)
Ending balance	10,857,362	9,514,220	8,858,768
GMDB | Annuity Contracts
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	387,993	376,894	321,217
Incurred guarantee benefits	16,999	48,832	95,747
Paid guarantee benefits	(25,942)	(38,661)	(34,021)
Changes in unrealized investment gains and losses	11,288	928	(6,049)
Ending balance	390,338	387,993	376,894
GMDB | Variable Life, Variable Universal Life & Universal Life
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	4,058,181	3,245,821	2,921,933
Incurred guarantee benefits	808,834	746,130	538,906
Paid guarantee benefits	(14,642)	(35,894)	(21,811)
Changes in unrealized investment gains and losses	144,937	102,124	(193,207)
Ending balance	4,997,310	4,058,181	3,245,821
GMIB | Annuity Contracts
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	27,038	30,618	39,681
Incurred guarantee benefits	(8,653)	(1,693)	(6,900)
Paid guarantee benefits	(1,377)	(1,892)	(1,938)
Changes in unrealized investment gains and losses	123	5	(225)
Ending balance	17,131	27,038	30,618
GMWB/GMIWB/ GMAB | Annuity Contracts
Movement in Liabilities for Guarantees on Long-Duration Contracts, Guaranteed Benefit Liability, Gross [Roll Forward]
Beginning balance	5,041,008	5,205,435	4,993,610
Incurred guarantee benefits	411,575	(164,427)	211,825
Paid guarantee benefits	0	0	0
Changes in unrealized investment gains and losses	0	0	0
Ending balance	$ 5,452,583	$ 5,041,008	$ 5,205,435