Revenue from Contracts with Customers - Unsatisfied Performance Obligations (Details) $ in Millions	6 Months Ended
	Jun. 30, 2018 USD ($)
Unsatisfied Performance Obligations
Revenue, Practical Expedient, Remaining Performance Obligation	true
Expected revenue to be recognized in future periods	$ 140.2
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	43.0
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	60.2
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	26.8
Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	10.2
Subscription services
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	133.1
Subscription services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	40.7
Subscription services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	56.5
Subscription services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	26.0
Subscription services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	9.9
Other business services
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	7.1
Other business services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2018-07-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	2.3
Other business services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2019-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	3.7
Other business services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	0.8
Other business services | Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2021-01-01
Unsatisfied Performance Obligations
Expected revenue to be recognized in future periods	$ 0.3