ACQUISITIONS - GRR Acquisition - Purchase price allocation (Details) - USD ($) $ in Thousands	Mar. 10, 2017	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Less: identified assets acquired and liabilities assumed
Goodwill		$ 273,421	$ 12,242	$ 150,771
Adjustment
Less: identified assets acquired and liabilities assumed
Goodwill	$ 1,000
GRR Acquisition
Consideration transferred
Cash paid	53,032
Class A Common Stock (274,998 shares)	5,500
Assumed liabilities	1,106
Total consideration transferred	59,638
Less: identified assets acquired and liabilities assumed
Working capital	7,728
Fixed assets	13,225
Total identifiable net assets acquired	47,589
Goodwill	12,049
Fair value allocated to net assets acquired	$ 59,638
GRR Acquisition | Class A Common Stock
Less: identified assets acquired and liabilities assumed
Shares issued in Merger Agreement	274,998
GRR Acquisition | Adjustment
Consideration transferred
Cash paid	$ 1,700
Assumed liabilities	1,100
Less: identified assets acquired and liabilities assumed
Working capital	1,700
GRR Acquisition | Customer relationships
Less: identified assets acquired and liabilities assumed
Intangible assets	21,484
GRR Acquisition | Customer relationships | Maximum | Adjustment
Less: identified assets acquired and liabilities assumed
Intangible assets	100
GRR Acquisition | Other
Less: identified assets acquired and liabilities assumed
Intangible assets	5,152
GRR Acquisition | Other | Maximum | Adjustment
Less: identified assets acquired and liabilities assumed
Intangible assets	$ 100