Summary of Significant Accounting Policies Earnings Per Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Income [Abstract]
Net income	$ 133.6	$ 126.8	$ 192.0	$ 190.1
Less: preferred stock dividend requirements and redemption premium	0.9	0.4	1.7	1.2
Earnings available for common shareholders	$ 132.7	$ 126.4	$ 190.3	$ 188.9
Common Shares Outstanding [Abstract]
Average number of common shares outstanding (in shares)	154.6	154.2	154.5	154.1
Add: effect of dilutive securities (in shares)	0.3	0.6	0.4	0.7
Diluted average number of common shares outstanding (in shares)	154.9	154.8	154.9	154.8
Earnings Per Share [Abstract]
Basic EPS	$ 0.86	$ 0.82	$ 1.23	$ 1.23
Diluted EPS	$ 0.86	$ 0.82	$ 1.23	$ 1.22