Schedule II - Valuation and Qualifying Accounts - Summary of the Deferred Tax Assets Valuation Allowance and Allowance for Doubtful Accounts Receivable (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Valuation Allowance of Deferred Tax Assets [Member]
Balance at beginning of year	$ 31,751	$ 27,616	$ 26,046
Additions	617	6,755	3,327
Deductions	25,126	2,620	1,757
Balance at end of year	7,242	31,751	27,616
Allowance for Doubtful Accounts, Current [Member]
Balance at beginning of year	21	4
Additions	14	21	4
Deductions	26	4
Balance at end of year	$ 9	$ 21	$ 4