Summary of Significant Accounting Policies (Impact of Adoption of Topic 606) (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Jan. 01, 2018	Dec. 31, 2017
Balance Sheet:
Accounts receivable, less allowance	$ 158.2			$ 148.2
Deferred commissions, current and non-current	26.1		$ 22.7
Deferred revenue, current and non-current	219.6
Income Statement:
Revenue	243.5	$ 209.5
Cost of revenue	102.4	97.0
Sales and marketing	38.5	32.4
Operating income	47.5	$ 28.4
Impact of adopting Topic 606 | ASU 2014-09
Balance Sheet:
Accounts receivable, less allowance	0.0
Deferred commissions, current and non-current	26.1		$ 22.7
Deferred revenue, current and non-current	0.0
Income Statement:
Revenue	1.7
Cost of revenue	1.7
Sales and marketing	0.4
Operating income	(0.4)
Balances without adoption of Topic 606
Balance Sheet:
Accounts receivable, less allowance	158.2
Deferred commissions, current and non-current	0.0			$ 0.0
Deferred revenue, current and non-current	219.6
Income Statement:
Revenue	241.8
Cost of revenue	100.7
Sales and marketing	38.9
Operating income	$ 47.1