Summary of Significant Accounting Policies - ASC 606 Impact on Balance Sheet (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Current assets:
Prepaid expenses and other	$ 19.1		$ 12.2
Total current assets	136.1		165.2
Other noncurrent assets	32.9		9.7
Total assets	2,250.8		2,441.6
Current liabilities
Current portion of unearned service revenue	50.1		43.2
Total current liabilities	204.6		191.4
Deferred income taxes, net	161.6		220.4
Other noncurrent liabilities	8.9		7.0
Total liabilities	2,650.1		2,646.0
Accumulated deficit	(638.3)		(500.4)
Total stockholders' deficit	(399.3)		(204.4)
Total liabilities and stockholders’ deficit	2,250.8		$ 2,441.6
ASU 2014-09 | Adjustments
Current assets:
Prepaid expenses and other	5.0
Total current assets	5.0
Other noncurrent assets	18.1
Deferred contract costs	23.1
Current liabilities
Current portion of unearned service revenue	3.5
Total current liabilities	3.5
Deferred income taxes, net	0.2
Other noncurrent liabilities	0.6
Total liabilities	4.3
Accumulated deficit	18.8	$ 9.1
Total stockholders' deficit	18.8
Total liabilities and stockholders’ deficit	23.1
ASU 2014-09 | Without adoption of Topic 606
Current assets:
Prepaid expenses and other	14.1
Total current assets	131.1
Other noncurrent assets	14.8
Total assets	2,227.7
Current liabilities
Current portion of unearned service revenue	46.6
Total current liabilities	201.1
Deferred income taxes, net	161.4
Other noncurrent liabilities	8.3
Total liabilities	2,645.8
Accumulated deficit	(657.1)
Total stockholders' deficit	(418.1)
Total liabilities and stockholders’ deficit	$ 2,227.7