Fair Value Measurements - Narrative (Details)	12 Months Ended
	Dec. 31, 2017
Minimum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Threshold period, past due for payment of services provided	15 days
Maximum
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Threshold period, past due for payment of services provided	60 days