Schedule II: Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	$ 95,635	$ 89,004	$ 268
Charged (credited) to expense	(27,808)	8,621	88,741
Deductions from reserves	(6,588)	(1,990)	(5)
Balance at end of period	61,239	95,635	89,004
Allowance for Doubtful Accounts [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	223	267	268
Charged (credited) to expense	0	(20)	4
Deductions from reserves	(47)	(24)	(5)
Balance at end of period	176	223	267
Valuation Allowance on Deferred Tax Assets [Member]
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at beginning of period	95,412	88,737	0
Charged (credited) to expense	(27,808)	8,641	88,737
Deductions from reserves	(6,541)	(1,966)	0
Balance at end of period	$ 61,063	$ 95,412	$ 88,737