Recently Issued Accounting Pronouncements (Impact of Adoption of Topic 606) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018
Balance Sheet:
Accounts receivable, less allowance for doubtful accounts	$ 172.2				$ 148.2				$ 172.2	$ 148.2
Deferred commissions, current and non-current	25.1				22.7				25.1	22.7		$ 22.7
Deferred revenue, current and non-current	210.0								210.0
Income Statement:
Revenue	262.7	$ 261.3	$ 252.4	$ 243.5	243.1	$ 229.9	$ 229.2	$ 209.5	1,019.9	911.7	$ 798.6
Cost of revenue									411.1	386.6	344.3
Sales and marketing									148.5	134.3	97.6
Operating income	49.3	65.4	$ 53.6	$ 47.5	42.6	$ 52.8	$ 46.0	$ 28.4	215.8	169.8	$ 180.8
Impact of adopting Topic 606
Balance Sheet:
Accounts receivable, less allowance for doubtful accounts		0.0
Deferred commissions, current and non-current	25.1								25.1
Deferred revenue, current and non-current		$ 0.0
Impact of adopting Topic 606 | ASU 2014-09
Balance Sheet:
Deferred commissions, current and non-current												$ 22.7
Income Statement:
Revenue									6.7
Cost of revenue									6.7
Sales and marketing									2.9
Operating income									(2.9)
Balances without adoption of Topic 606
Balance Sheet:
Accounts receivable, less allowance for doubtful accounts	172.2								172.2
Deferred commissions, current and non-current	0.0				$ 0.0				0.0	$ 0.0
Deferred revenue, current and non-current	$ 210.0								210.0
Income Statement:
Revenue									1,013.2
Cost of revenue									404.4
Sales and marketing									151.4
Operating income									$ 212.9