Summary of Significant Accounting Policies (Narrative) (Details) - Accounting Standards Update 2016-09 [Member] - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2017	Jun. 30, 2016
Summary of Significant Accounting Policies [Line Items]
Excess tax benefits recognized	$ 5	$ 25
Excess Tax Benefits [Member]
Summary of Significant Accounting Policies [Line Items]
Decrease in net cash used for operating activities and net cash provided by financing activities			$ 18
Employee Taxes Paid [Member]
Summary of Significant Accounting Policies [Line Items]
Decrease in net cash used for operating activities and net cash provided by financing activities			$ 25