Equity Method Investments - Activity Related to Equity Method Investments (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Equity Method Investments
Balance at beginning of period	$ 103,517	$ 68,029	$ 26,892
Contributions	28,484	29,073	38,057
Earnings (loss) on equity method investments	(2,477)	6,415	3,080
Balance at end of period	129,524	103,517	68,029
Constitution Pipeline Company, LLC
Equity Method Investments
Balance at beginning of period	90,345	64,268	26,892
Contributions	8,975	19,625	34,200
Earnings (loss) on equity method investments	(2,470)	6,452	3,176
Balance at end of period	96,850	90,345	64,268
Meade Pipeline Co LLC
Equity Method Investments
Balance at beginning of period	13,172	3,761	0
Contributions	19,509	9,448	3,857
Earnings (loss) on equity method investments	(7)	(37)	(96)
Balance at end of period	$ 32,674	$ 13,172	$ 3,761