Recent Accounting Standards Recent Accounting Standards (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Accounting Policies [Abstract]
Net periodic costs excluding service costs	$ 9.6	$ 3.0	$ 2.9
Restricted cash and cash equivalents	6.8	$ 2.6	$ 3.2	$ 3.8
Increase in third-party costs due to revenue standard low range	1,100.0
Increase in third-party costs due to revenue standard high range	$ 1,300.0