Related Party Transactions - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Oak Valley Management Limited Liability Company [Member]
Related Party Transaction [Line Items]
General and administrative expenses related to services	$ 0.8	$ 0.8	$ 3.0	$ 2.9
Oak Valley Resources, Limited Liability Company [Member]
Related Party Transaction [Line Items]
Accounts payable, related parties	$ 1.0		$ 1.0		$ 2.3