Discontinued Operations - Additional Information (Detail) (USD $)	1 Months Ended	3 Months Ended	6 Months Ended	9 Months Ended			1 Months Ended
	Jul. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Sep. 30, 2013	Sep. 30, 2012		Aug. 30, 2013 Murphy USA Inc [Member]
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
After tax gain on sale of assets				$ 216,200,000
Cash dividends paid prior to spin-off				650,000,000	0	[1]	650,000,000
United kingdom, corporation tax rate, tax relief on oil and gas decommissioning costs	50.00%		62.00%
United kingdom, increase in tax expense of discontinued operations		$ 5,500,000			$ 5,500,000

[1] Reclassified to conform to current presentation.