Contingencies, Commitments and Guarantees (Insolvency Assessments) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Other Assets:
Premium tax offset for future discounted and undiscounted assessments	$ 56	$ 30
Premium tax offsets currently available for paid assessments	50	33
Other Liabilities:
Insolvency assessments	75	47
Insurance-related Assessments
Loss Contingencies [Line Items]
Total assets held for insolvency assessments	$ 106	$ 63