Equity (Issuance of Stock and Forward Sale Agreement) (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended		12 Months Ended
	Nov. 30, 2017	Nov. 30, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Stock [Line Items]
Antidilutive securities (in shares)			100,000	3,100,000	7,900,000
Forward sale price (in dollars per share)		$ 124.00
Common Stock [Member]
Class of Stock [Line Items]
Stock sold during the period (in shares)			6,000,000	2,000,000	6,000,000
Common Stock [Member] | Forward Counterparty [Member]
Class of Stock [Line Items]
Settlement of shares (in shares)		12,000,000
Stock sold during the period (in shares)				1,711,345
NEP [Member]
Class of Stock [Line Items]
Senior unsecured convertible notes outstanding			$ 300
Series A Convertible Preferred Units [Member] | NEP [Member]
Class of Stock [Line Items]
Stock sold during the period	$ 550