Earnings per Common Share - Textual (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions						1 Months Ended						3 Months Ended		9 Months Ended
	Oct. 20, 2017	Oct. 16, 2017	Jul. 20, 2017	Oct. 20, 2016	Sep. 30, 2009	Jul. 31, 2017	May 31, 2017	Apr. 30, 2017	Feb. 28, 2017	Jan. 31, 2017	Jan. 17, 2017	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Cash dividends per common share, Declared (in dollars per share)			$ 0.25	$ 0.24								$ 0.25	$ 0.24	$ 0.73	$ 0.70
Stock warrants exercised (shares)					5.0						0.7
Class of Warrant or Right, Outstanding					8.2						0.0					8.2
Exercise price of warrants (in dollars per share)					$ 41.40						$ 41.40					$ 41.40
Proceeds from exercising stock warrants											$ 27.0			$ 26.6	$ 0.0
Common Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Cash dividends per common share, Declared (in dollars per share)							$ 0.24		$ 0.24
Cash dividends per common share, Paid (in dollars per share)						$ 0.24		$ 0.24		$ 0.24
Common stock dividends accrued												$ 25.3		$ 25.3		$ 22.2
Subsequent Event [Member] | Common Stock
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Cash dividends per common share, Declared (in dollars per share)	$ 0.25
Cash dividends per common share, Paid (in dollars per share)		$ 0.25