Level 4~Investments - Other Than Temporary Impairment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Credit impairments	$ 0	$ 1
Impairments on equity securities and other impairments	0	1
Intent-to-sell impairments	0	0
Cumulative Credit Impairments
Beginning balance	(25)	(110)
Additions for credit impairments recognized on:
Securities not previously impaired	0	(1)
Reductions for credit impairments previously recognized on:
Securities that matured or were sold during the period	4	7
Securities due to an increase in expected cash flows	0	5
Ending balance	(21)	(99)
Other than Temporary Impairment Losses, Investments	0	1
Equity securities
Other than Temporary Impairment, Credit Losses Recognized in Earnings [Line Items]
Impairments on equity securities and other impairments	$ 0	$ 0