Equity Method Investments - Contributions to and Earnings (Loss) on the Company's Equity Method Investments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Schedule of Equity Method Investments [Line Items]
Contributions	$ 5,036	$ 16,293	$ 10,114	$ 22,230
Earnings (loss) on equity method investments	1,512	756	2,933	756
Contributions to and earnings (loss) on equity method investments	6,548	17,049	13,047	22,986
Constitution Pipeline Company, LLC
Schedule of Equity Method Investments [Line Items]
Contributions	3,000	15,250	6,000	21,000
Earnings (loss) on equity method investments	1,528	854	2,955	854
Contributions to and earnings (loss) on equity method investments	4,528	16,104	8,955	21,854
Meade Pipeline Co LLC
Schedule of Equity Method Investments [Line Items]
Contributions	2,036	1,043	4,114	1,230
Earnings (loss) on equity method investments	(16)	(98)	(22)	(98)
Contributions to and earnings (loss) on equity method investments	$ 2,020	$ 945	$ 4,092	$ 1,132