Business Segment Information - Major Customers (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Revenue, Major Customer [Line Items]
Sales	$ 346,881	$ 394,107	$ 374,714	$ 443,056	$ 465,108	$ 493,603	$ 512,987	$ 511,690	$ 1,558,758	$ 1,983,388	$ 1,934,642
Plains Marketing, L.P.
Revenue, Major Customer [Line Items]
Sales									597,117	491,258	426,339
Percentage of revenue									38.30%	24.80%	15.60%
Shell Trading (US) Company
Revenue, Major Customer [Line Items]
Sales										347,422
Percentage of revenue										17.50%
Atlas Pipeline Mid-Continent West OK LLC
Revenue, Major Customer [Line Items]
Sales									333,027	211,838
Percentage of revenue									21.40%	10.70%
Enterprise Crude Oil, LLC
Revenue, Major Customer [Line Items]
Sales											$ 394,162
Percentage of revenue											14.40%