Related Party Transactions - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Oak Valley Management Limited Liability Company [Member]
Related Party Transaction [Line Items]
General and administrative expenses related to services	$ 3.8	$ 3.9	$ 2.2
Oak Valley Resources, Limited Liability Company [Member]
Related Party Transaction [Line Items]
Accounts payable, related parties	$ 0.7	$ 2.3