Schedule II - Valuation and Qualifying Accounts - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts [Member]
Allowance for doubtful accounts		$ 3,163	$ 1,794	$ 1,667
Allowance for doubtful accounts		2,400	1,917	606
Allowance for doubtful accounts	[1]	0	167	0
Allowance for doubtful accounts	[2]	(957)	(715)	(479)
Allowance for doubtful accounts		4,606	3,163	1,794
Valuation Allowance of Deferred Tax Assets [Member]
Allowance for doubtful accounts		1,532	1,683	2,052
Allowance for doubtful accounts		(1,457)	0	0
Allowance for doubtful accounts	[1]	0	0	0
Allowance for doubtful accounts	[2]	0	(151)	(369)
Allowance for doubtful accounts		$ 75	$ 1,532	$ 1,683

[1] In 2016, the change in the allowance for doubtful accounts represents the fair value of balances assumed in acquisition transactions. See Note 2 "Acquisitions and Dispositions" of the notes to our audited consolidated financial statements included elsewhere herein for further information.

[2] Deductions from allowance for doubtful accounts represent write-offs of receivable balances not considered collectible. The deduction from the valuation allowance for deferred tax assets represents changes in estimates of our future taxable income and our estimated future usage of certain net operating loss carryforwards, as well as expiration of certain net operating loss carryforwards.