Commitments and Contingencies (Details) - USD ($) $ in Millions	1 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Loss Contingencies [Line Items]
Undrawn Letters Of Credit	$ 72	$ 72
Operating Leases, Rent Expense		710	$ 608	$ 555
Surety Bonds Outstanding	1,400	$ 1,400
Payments to be made to Acquire Businesses, Gross	$ 7,700