Significant Accounting Policies - Balance Sheet Impact of New Accounting Pronouncements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Jan. 01, 2018	Dec. 31, 2017
ASSETS
Accounts receivable, net	$ 28.8	$ 27.8	$ 49.6
Income taxes receivable	17.0	31.5	35.6
Current assets of discontinued operations held for sale	0.0	69.8	69.1
Other assets	16.1	12.5	13.6
LIABILITIES
Accrued expense	89.8	76.6	75.8
Current deferred revenue	47.9	52.0	70.9
Current liabilities of discontinued operations held for sale	0.0	149.4	188.2
Non-current deferred revenue	21.1	24.8	29.3
Deferred income taxes	78.2	40.5	40.6
Non-current liabilities of discontinued operations held for sale	0.0	60.3	54.8
SHAREHOLDERS' EQUITY
Retained earnings	474.2	664.0	$ 634.3
Balances without Adoption of ASC 606
ASSETS
Accounts receivable, net	53.7
Other assets	16.7
LIABILITIES
Accrued expense	88.8
Current deferred revenue	70.1
Non-current deferred revenue	25.2
Deferred income taxes	78.0
SHAREHOLDERS' EQUITY
Retained earnings	474.6
Accounting Standards Update 2014-09 | Difference between Revenue Guidance in Effect before and after Topic 606
ASSETS
Accounts receivable, net	(24.9)	(21.8)
Income taxes receivable		(4.1)
Current assets of discontinued operations held for sale		0.7
Other assets	(0.6)	(1.1)
LIABILITIES
Accrued expense	1.0	0.8
Current deferred revenue	(22.2)	(18.9)
Current liabilities of discontinued operations held for sale		(38.8)
Non-current deferred revenue	(4.1)	(4.5)
Deferred income taxes	0.2	(0.1)
Non-current liabilities of discontinued operations held for sale		5.5
SHAREHOLDERS' EQUITY
Retained earnings	$ (0.4)	$ 29.7