Revenue from Contracts with Customers - Narrative (Details)	6 Months Ended
	Jun. 30, 2019
Disaggregation of Revenue [Line Items]
Contract terms	Generally, contracts are billed in arrears on a quarterly basis and have a three year duration, after which the customer can terminate the agreement with a three to twelve month notice.
Minimum
Disaggregation of Revenue [Line Items]
Termination period	3 months
Maximum
Disaggregation of Revenue [Line Items]
Termination period	12 months