Schedule II - Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Deferred Income Tax Valuation Allowance
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at the beginning of the year	$ 99,638	$ 118,957	$ 7,150
Charged to costs and expenses	0	0	111,807
Charged to other accounts	0	0	0
Other additions (deductions)	(47,628)	(19,319)	0
Balance at the end of the year	52,010	99,638	118,957
Reserve for Materials and Supplies
Valuation And Qualifying Accounts Disclosure [Line Items]
Balance at the beginning of the year	956	988	1,101
Charged to costs and expenses	0	0	0
Charged to other accounts	0	0	0
Other additions (deductions)	138	(32)	(113)
Balance at the end of the year	$ 1,094	$ 956	$ 988