Recent Accounting Pronouncements New Accounting Standards - Effect of change related to ASU 2014-09 (Details) - USD ($) $ in Millions	Jun. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets [Abstract]
Accounts receivable, net	$ 37.3	$ 27.8	$ 49.6
Other assets	(13.1)	(12.5)	(13.6)
Liabilities [Abstract]
Accrued expense	95.2	76.6	75.8
Income taxes payable	24.5		0.0
Deferred revenue	(13.6)	(52.0)	(70.9)
Deferred revenue	(21.1)	(24.8)	(29.3)
Deferred income taxes	48.6	40.5	40.6
Equity [Abstract]
Retained earnings	448.1	664.0	$ 634.3
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Assets [Abstract]
Accounts receivable, net	0.7	21.8
Other assets	(1.8)	(1.1)
Liabilities [Abstract]
Accrued expense	0.8	0.8
Income taxes payable	0.2
Deferred revenue	(1.8)	(18.9)
Deferred revenue	(2.5)	(4.5)
Deferred income taxes	0.2	0.1
Equity [Abstract]
Retained earnings	0.6	$ 29.7
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets [Abstract]
Accounts receivable, net	38.0
Other assets	(14.9)
Liabilities [Abstract]
Accrued expense	94.4
Income taxes payable	24.3
Deferred revenue	(15.4)
Deferred revenue	(23.6)
Deferred income taxes	48.4
Equity [Abstract]
Retained earnings	$ 447.5