Reinsurance Reinsurance (Net Amount at Risk and 100 Percent Mortality) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Guaranteed Minimum Benefits [Line Items]
Average attained age of all policyholders under all benefits reinsured, years	70 years
GLB
Guaranteed Minimum Benefits [Line Items]
Net Amount at Risk	$ 691	$ 800
GMDB
Guaranteed Minimum Benefits [Line Items]
Net Amount at Risk	$ 279	341
Mortality percentage according to Annuity 2000 mortality table	100.00%
GMDB with Both Risk(GMDB and GLB) [Member]
Guaranteed Minimum Benefits [Line Items]
Net Amount at Risk	$ 81	88
GLB with Both Risk(GLB and GMDB) [Member]
Guaranteed Minimum Benefits [Line Items]
Net Amount at Risk	$ 392	$ 464
Minimum | GLB
Guaranteed Minimum Benefits [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Discount Rate	4.25%
Minimum | GMDB
Guaranteed Minimum Benefits [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Discount Rate	4.00%
Minimum | GLB with Both Risk(GLB and GMDB) [Member]
Guaranteed Minimum Benefits [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Discount Rate	4.25%
Maximum | GLB
Guaranteed Minimum Benefits [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Discount Rate	4.75%
Maximum | GMDB
Guaranteed Minimum Benefits [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Discount Rate	4.50%
Maximum | GLB with Both Risk(GLB and GMDB) [Member]
Guaranteed Minimum Benefits [Line Items]
Long-Duration Contracts, Assumptions by Product and Guarantee, Discount Rate	4.75%