Dividends Dividends (Details) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended
	Feb. 28, 2017	Feb. 29, 2016	Sep. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Dec. 31, 2015
Payments of Ordinary Dividends, Common Stock	$ 395.4	$ 519.0
Dividends payable			$ 0.0	$ 395.4	$ 0.0	$ 519.2
Annual variable dividend [Member]
Common Stock, Dividends, Per Share, Cash Paid	$ 0.6808	$ 0.8882