Equity Method Investments - Schedule of Equity Investments in Affiliates Balance Affected (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Schedule of Equity Method Investments [Line Items]
Distributions received	$ 117,900,000	$ 76,300,000	$ 8,300,000
Income from equity method investments	101,100,000	70,100,000	6,700,000
Mars
Schedule of Equity Method Investments [Line Items]
Distributions received	52,200,000	34,900,000	5,500,000
Income from equity method investments	47,000,000	33,200,000	3,400,000
Amortization expense (income)	1,100,000	1,000,000	100,000
Bengal
Schedule of Equity Method Investments [Line Items]
Distributions received	19,600,000	20,200,000	2,800,000
Income from equity method investments	20,200,000	20,800,000	3,300,000
Amortization expense (income)	(200,000)	(300,000)	$ 0
Proteus and Endymion
Schedule of Equity Method Investments [Line Items]
Income from equity method investments	100,000
Poseidon
Schedule of Equity Method Investments [Line Items]
Distributions received	41,900,000	21,200,000
Income from equity method investments	29,700,000	16,100,000
Amortization expense (income)	900,000	$ 400,000
Odyssey
Schedule of Equity Method Investments [Line Items]
Distributions received	4,200,000
Income from equity method investments	4,200,000
Amortization expense (income)	100,000
Proteus
Schedule of Equity Method Investments [Line Items]
Income from equity method investments	100,000
Amortization expense (income)	100,000.0
Endymion
Schedule of Equity Method Investments [Line Items]
Income from equity method investments	100,000
Amortization expense (income)	$ 100,000.0