Accounts Receivable Grants from Outside Parties (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Nontrade Receivables [Abstract]
Grant proceeds received from outside parties	$ 6,495	$ 5,934
Amortization of deferred grants included as offset to depreciation expense	$ 3,508	$ 2,467