Accounts Receivable Grants from Outside Parties (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Grant Proceeds From Outside Parties	$ 20,249	$ 36,094	$ 41,742
Amortization of deferred grants included as offset to depreciation expense	$ 12,356	$ 13,465	$ 10,691