Equity - Common Stock Narrative (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions			1 Months Ended	3 Months Ended								12 Months Ended
	Mar. 02, 2018	Feb. 01, 2018	Jun. 30, 2015	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Subsequent Event [Line Items]
Dividends declared per common share (in dollars per share)												$ 1.48	$ 2.095	$ 2.260
Dividends paid per common share (in dollars per share)				$ 0.37	$ 0.37	$ 0.37	$ 0.37	$ 0.37	$ 0.58	$ 0.58	$ 0.58	$ 1.480	$ 2.095	$ 2.260
Subsequent Event
Subsequent Event [Line Items]
Dividends declared per common share (in dollars per share)		$ 0.37
Dividends paid per common share (in dollars per share)	$ 0.37
At-The-Market Program
Subsequent Event [Line Items]
Maximum sales under equity offering program			$ 750
Common stock issued (in shares)														1.8
Weighted average price (in dollars per share)														$ 40.14
Proceeds from issuance of common stock														$ 73
Stock issuance fees and commissions														$ 1