Equity (Issuance of Common Stock and Forward Sale Agreement) (Details) - $ / shares	1 Months Ended	12 Months Ended
	Nov. 30, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class of Stock [Line Items]
Antidilutive securities (in shares)		3,100,000	7,900,000	3,500,000
Forward sale price (in dollars per share)	$ 124.00
Common Stock [Member]
Class of Stock [Line Items]
Stock sold during the period (in shares)		2,000,000	6,000,000	17,000,000
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