Derivative Financial Instruments - Summary of Gain (Loss) Recognized in Income on Oil and Gas Commodity Contracts on (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended		15 Months Ended	3 Months Ended	9 Months Ended		15 Months Ended
	Jun. 30, 2013	Jun. 30, 2013	Jul. 01, 2012	Jul. 01, 2013	Jun. 30, 2013 Commodity contracts [Member] Other Income Expense [Member]	Jun. 30, 2013 Commodity contracts [Member] Other Income Expense [Member]	Jul. 01, 2012 Commodity contracts [Member] Other Income Expense [Member]	Jul. 01, 2013 Commodity contracts [Member] Other Income Expense [Member]	Feb. 14, 2013 Swap [Member] Natural Gas Commodity Contract [Member] MMBTU	Jun. 30, 2013 Remainder of 2013 [Member] Swap [Member] Natural Gas Commodity Contract [Member]	Jun. 30, 2013 Remainder of 2013 [Member] Swap [Member] Oil Commodity Contract [Member]	Feb. 14, 2013 Remainder of 2013 [Member] Swap [Member] Oil Commodity Contract [Member] MBbls	Jun. 30, 2013 2014 [Member] Swap [Member] Natural Gas Commodity Contract [Member]	Jun. 30, 2013 2014 [Member] Swap [Member] Oil Commodity Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Document Period End Date		Jun. 30, 2013
Derivative, Average Forward Price										95.12	3.64		89.97	3.91
Derivative, mitigating price volatility, natural gas covered									13,365,300,000
Derivative, mitigating price volatility, oil covered												408,000
Cash settlements on derivative financial instruments					$ (1.9)	$ (1.3)
Non-cash change in fair value of derivative financial instruments					11.5	2.1
Gain on oil and natural gas commodity contracts	$ 136.2	$ 172.2	$ (119.4)	$ (158.1)	$ 9.6	$ 0.8	$ 0	$ 0