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 [Roll Forward]
Balance at beginning of year	$ 171	$ 171	$ 171
Addition charged to costs and expenses	0	0	10
Other additions (deductions)	0	0	0
Less deductions	0	0	0
Balance at end of year	171	171	161
Deferred tax asset valuation allowance
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of year	71,208	88,934	86,092
Addition charged to costs and expenses	17,492	0	0
Other additions (deductions)	0	2,842	13,988
Less deductions	(35,218)	0	0
Balance at end of year	$ 88,934	$ 86,092	$ 72,104