Equity and Share-Based Compensation (Details) (USD $)	1 Months Ended	3 Months Ended			1 Months Ended
	Apr. 30, 2012	Jun. 30, 2012 item	Apr. 25, 2012	Dec. 31, 2011 Common units	Apr. 30, 2012 Petroleum Inc.	Mar. 31, 2011 Petroleum Inc. Common units	Apr. 30, 2012 Petroleum Inc. Common units Chief executive officer of MPCI	Mar. 31, 2011 Petroleum Inc. Common units Chief executive officer of MPCI
Transactions with Chief Executive Officer of Petroleum Inc.
Units issued and outstanding				256,742
Common stock issued for each unit upon conversion (in shares)	185.5				18,762
Common stock issued upon conversion (in shares)	47,634,353
Number of shares of common stock sold	18,000,000							17.3
Initial public offering, aggregate number of shares of common stock sold	27,600,000
Common stock, par value (in dollars per share)		$ 0.01	$ 0.01
Number of shares of stock sold by the selling shareholders	6,000,000
Number of shares of common stock sold by the selling stockholders pursuant to an option granted to the underwriters to cover over-allotments	3,600,000
Net proceeds from registration and sale	$ 220,000,000
Common stock, shares authorized		300,000,000	300,000,000
Number of votes per share entitled to holders of common shares		1
Preferred stock, shares authorized		50,000,000	50,000,000
Preferred stock, par value (in dollars per share)		$ 0.01	$ 0.01
Common stock, shares issued		66,549,563	66,549,563
Common stock, shares outstanding		66,549,563
Shares granted to Chief Executive Officer								24.6
Grant date fair value (in dollars per share)						$ 80,013	$ 4.26
Grant date fair value (in dollars)						$ 3,400,000