Reinsurance - Account Value, Net Amount at Risk and Contractholders for GMDB Business (Details) - Variable Annuity [Member] - Guaranteed Minimum Death Benefit [Member] Contractholders in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018 USD ($) Contractholders	Dec. 31, 2017 USD ($) Contractholders
Guaranteed Minimum Death Benefits Account Value, Net Amount at Risk And Average Age Table [Line Items]
Account value	$ 9,762	$ 10,109
Net amount at risk	$ 2,120	$ 2,112
Number of contractholders | Contractholders	240,000	245,000