Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred income tax assets:
Capital and financing lease obligations	$ 264,255	$ 862,038
Operating loss carryforwards	288,469	319,948
Accrued expenses	66,123	109,283
Deferred lease liability	52,869	93,358
Tax credits	49,556	49,550
Intangible assets	14,493	20,272
Deferred gain on sale leaseback	1,844	4,233
Prepaid revenue	1,825	2,626
Investment in unconsolidated ventures	2,861	0
Other	660	0
Total gross deferred income tax asset	742,955	1,461,308
Valuation allowance	(336,087)	(264,305)
Net deferred income tax assets	406,868	1,197,003
Deferred income tax liabilities:
Property, plant and equipment	(477,512)	(1,209,595)
Investment in unconsolidated ventures	0	(66,678)
Other	0	(1,376)
Total gross deferred income tax liability	(477,512)	(1,277,649)
Net deferred tax liability	$ (70,644)	$ (80,646)