Business Combinations (Narrative) (Details) (USD $)	3 Months Ended		6 Months Ended		12 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Jun. 30, 2013 Acquisition Field Revenue [Member]	Jun. 30, 2013 Acquisition Field Revenue [Member]	Jun. 30, 2013 COP Acquisition [Member]	Jun. 30, 2013 Alba Field [Member]	May 31, 2012 Alba Field [Member]
Business Acquisition [Line Items]
Acquisition date								Dec. 23, 2011
Effective date of acquisition									May 31, 2012
Interest acquired, percentage									23.43%
Interest percentage in entity									25.68%
Cash paid										$ 229,577,000
Acquisition field revenue	126,165,000	23,003,000	183,837,000	38,169,000		94,900,000	116,200,000
Acquisition income from operations	31,257,000	(18,356,000)	39,476,000	(37,057,000)		11,500,000	15,000,000
Deposit amount					$ 10,000,000