Equity Method Investments - Activity Related to Equity Method Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Equity Method Investments
Balance at beginning of period	$ 129,524	$ 103,517	$ 68,029
Contributions	57,039	28,484	29,073
Earnings (loss) on equity method investments	(100,486)	(2,477)	6,415
Balance at end of period	86,077	129,524	103,517
Constitution Pipeline Company, LLC
Equity Method Investments
Balance at beginning of period	96,850	90,345	64,268
Contributions	4,350	8,975	19,625
Earnings (loss) on equity method investments	(100,468)	(2,470)	6,452
Balance at end of period	732	96,850	90,345
Meade Pipeline Co LLC
Equity Method Investments
Balance at beginning of period	32,674	13,172	3,761
Contributions	52,689	19,509	9,448
Earnings (loss) on equity method investments	(18)	(7)	(37)
Balance at end of period	$ 85,345	$ 32,674	$ 13,172