Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for doubtful accounts
Movement in Valuation Allowances and Reserves
Balance at beginning of period	$ 420	$ 291	$ 425
Additions charged to costs and expenses	181	109	92
Additions charged to other accounts	33	19	(61)
Deductions	(90)	1	(165)
Balance at end of period	544	420	291
Valuation allowance on deferred tax assets
Movement in Valuation Allowances and Reserves
Balance at beginning of period	70,241	69,928	91,462
Additions charged to costs and expenses	2,215	4,876	16,320
Additions charged to other accounts	(23,119)	(4,563)	(37,854)
Deductions	0	0	0
Balance at end of period	$ 49,337	$ 70,241	$ 69,928