Equity Investments - Narrative (Details)	1 Months Ended			12 Months Ended
	Feb. 29, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2004 USD ($)	Dec. 31, 2017 USD ($) ft	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule of Equity Method Investments [Line Items]
Loss from equity method investments				$ 2,368,000	$ 2,166,000	$ 124,345,000
Investee losses in excess of investment				7,567,000	10,238,000
Distributions from equity investments				0	1,200,000	7,000,000
Sale of ownership interest for cash				$ 0	25,000,000	0
Summarized financial information, revenue						14,800,000
Summarized financial information, operating loss						(448,100,000)
Summarized financial information, net loss						$ (448,100,000)
Independence Hub, LLC [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest		20.00%		20.00%		20.00%
Water depth (in feet) | ft				8,000
Impairment charge		$ 343,300,000
Estimated fair value		0				$ 0
Loss from equity method investments				$ 2,400,000	2,200,000	74,900,000
Capitalized interest		$ 3,600,000				3,600,000
Participation fee			$ 1,000,000
Investee losses in excess of investment				$ 9,800,000	$ 10,200,000
Distributions from equity investments						$ 1,800,000
Deepwater Gateway, L.L.C. [Member]
Schedule of Equity Method Investments [Line Items]
Percentage of ownership interest		50.00%				50.00%
Impairment charge		$ 96,700,000
Estimated fair value		70,800,000				$ 70,800,000
Loss from equity method investments						49,400,000
Capitalized interest		$ 1,200,000				1,200,000
Distributions from equity investments	$ 1,200,000					$ 5,200,000
Sale of ownership interest for cash	$ 25,000,000