Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Equity [Abstract]
Stock repurchased (in shares)				23.1		17.6
Stock repurchased and retired (in shares)				5.5
Stock repurchased and retired (in dollars per share)				$ 20.98
Common stock issued under stock compensation plan (in shares)				4.9
Net decrease in period (in shares)				18.2
Increase in quarterly dividend rate (in dollars per share)		$ 0.01
Dividends declared (in dollars per share)	$ 0.11		$ 0.10	$ 0.33	$ 0.29
Dividends declared	$ 28.8