Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Contingency [Line Items]
Tax reform, provisional charge		$ 275
Tax benefit from foreign Supreme Court decision			$ 30
Tax benefit from prior year tax positions currently being challenged	$ 15
Accounting Standards Update 2018-02
Income Tax Contingency [Line Items]
Tax reform, reclassification from AOCI to retained earnings, tax effect	$ 163