Summary of Significant Accounting Policies, Recent Accounting Pronouncements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Jan. 01, 2017
New Accounting Pronouncements
Income tax expense (benefit)					$ 691.7	$ 308.5	$ 280.5
Impact of ASU 2016-09
New Accounting Pronouncements
Income tax expense (benefit)	$ (20.8)	$ (16.6)	$ (21.2)	$ (8.0)	$ (66.6)
Increase in diluted earnings per share (in dollars per share)					$ 0.21
Cumulative-effect adjustment to retained earnings								$ 0.0