Equity (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Equity [Abstract]
Stock repurchased (in shares)			15.6		10.1
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)			4.2
Net decrease in period (in shares)			11.4
Increase in quarterly dividend rate (in dollars per share)	$ 0.01
Dividends declared (in dollars per share)	$ 0.11	$ 0.10	$ 0.22	$ 0.19
Dividends declared	$ 29.6