Summary of Significant Accounting Policies Summary of Significant Accounting Policies (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Drilling and Related Costs Capitalized	$ 11,700	$ 12,900
Impairment of long-lived assets	0	4,446	$ 246,625
Write-downs	3,390
Held-to-maturity Securities, Restricted	$ 20,800	$ 17,600