Recent Accounting Pronouncements New Accounting Standards - Effect of change related to ASU 2014-09 (Details) - USD ($) $ in Millions	Mar. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
Assets [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $3.9 at March 31, 2018 and $3.6 at December 31, 2017	$ (34.6)	$ (27.8)	$ (49.6)
Other assets	(12.5)	(12.5)	(13.6)
Liabilities [Abstract]
Accrued expense	75.6	76.6	75.8
Deferred revenue	(91.5)	(52.0)	(70.9)
Deferred revenue	(25.0)	(24.8)	(29.3)
Deferred income taxes	43.8	40.7	40.6
Equity [Abstract]
Retained earnings	344.5	663.4	$ 634.3
Difference between Revenue Guidance in Effect before and after Topic 606 [Member]
Assets [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $3.9 at March 31, 2018 and $3.6 at December 31, 2017	(3.1)	(21.8)
Other assets	(2.0)	(1.1)
Liabilities [Abstract]
Accrued expense	0.8	0.8
Deferred revenue	(0.2)	(18.9)
Deferred revenue	(4.3)	(4.5)
Deferred income taxes	0.1	0.1
Equity [Abstract]
Retained earnings	(1.5)	$ 29.1
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
Assets [Abstract]
Accounts receivable, net of allowance for doubtful accounts of $3.9 at March 31, 2018 and $3.6 at December 31, 2017	(37.7)
Other assets	(14.5)
Liabilities [Abstract]
Accrued expense	74.8
Deferred revenue	(91.7)
Deferred revenue	(29.3)
Deferred income taxes	43.7
Equity [Abstract]
Retained earnings	$ 346.0