General Adoption of New Accounting Pronouncements (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Effective Income Tax Rate Reconciliation, Change in Enacted Tax Rate, Amount		$ 450
Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative Effect on Retained Earnings, Tax	$ 7
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	$ 454
Accumulated Net Investment Gain (Loss) Attributable to Parent [Member] | Adjustments for New Accounting Pronouncement [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent			$ 416