Fair Value Measurements - Narrative (Details) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent consideration – current year acquisitions	$ 983,000
Remeasurement of Fair Value	0
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements	1,380,000
Consideration liability balance	54,272,000	$ 48,362,000
Acquisition consideration
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Contingent consideration – current year acquisitions	$ 12,807,000	46,800,000
Remeasurement of Fair Value		1,800,000
Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Liability, Settlements		0
Consideration liability balance		$ 45,000,000