Fair Value Measurements (Narrative) (Details) - USD ($)	3 Months Ended
	Jun. 30, 2017	Mar. 31, 2017
Fair Value Disclosures [Abstract]
Transfers between fair value measurement levels	$ 0
Level 2 (Significant other observable inputs):
Fair Value Inputs, Liabilities, Quantitative Information [Line Items]
Fair value of debt	$ 6,400,000,000	$ 8,300,000,000