Summary of Significant Accounting Policies - Asset Retirement Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Asset retirement obligations
Asset retirement obligation as of beginning of period	$ 168,142	$ 140,086
Accretion expense	9,696	10,646
Liabilities incurred	2,943	2,251
Liabilities settled	(1,484)	(5,027)
Change in estimates	64,303	20,186
Asset retirement obligation as of end of period	$ 243,600	$ 168,142