Long-Term Debt and Financing Arrangements - Additional Information (Detail) (USD $)	6 Months Ended															6 Months Ended
	Jun. 30, 2012	Dec. 31, 2011	Mar. 31, 2012 Notes payable due 2012 Fixed-to-Floating Interest Rate Swaps Terminated	Jan. 31, 2012 Notes payable due 2012 Fixed-to-Floating Interest Rate Swaps Terminated	Jun. 30, 2012 Notes payable due 2013 Fixed-to-Floating Interest Rate Swaps Terminated	Jan. 31, 2012 Notes payable due 2013 Fixed-to-Floating Interest Rate Swaps Terminated	Jun. 30, 2012 Notes payable due 2014 Fixed-to-Floating Interest Rate Swaps Terminated	Jan. 31, 2012 Notes payable due 2014 Fixed-to-Floating Interest Rate Swaps Terminated	Jun. 30, 2012 Notes payable due 2016 Fixed-to-Floating Interest Rate Swaps Terminated	Jan. 31, 2012 Notes payable due 2016 Fixed-to-Floating Interest Rate Swaps Terminated	Jun. 30, 2012 Notes payable due 2021 Fixed to Floating Interest Rate Swap	Jan. 31, 2012 Notes payable due 2021 Fixed to Floating Interest Rate Swap	Dec. 31, 2011 Notes payable due 2021 Fixed to Floating Interest Rate Swap	Jun. 30, 2012 Notes payable due 2028 Fixed to Floating Interest Rate Swap	Jan. 31, 2012 Notes payable due 2028 Fixed to Floating Interest Rate Swap	Jun. 30, 2012 Convertible notes payable due in 2012	Jun. 30, 2012 BondHedge [Member]
Debt Instrument [Line Items]
Stock Issued During Period, Shares, Conversion of Convertible Securities	640,018																640,772
Debt instrument, face amount				$ 200,000,000		$ 250,000,000		$ 300,000,000		$ 300,000,000	$ 400,000,000	$ 200,000,000	$ 200,000,000		$ 150,000,000	$ 320,000,000
Unamortized gain on terminated swap	17,300,000		1,700,000		7,100,000		4,700,000		12,300,000		400,000			6,200,000
Unamortized debt discount					100,000
Long-term debt fair value adjustment							7,300,000		18,000,000					17,400,000
Debt instrument, maturity date																May 17, 2012
Conversion rate of convertible notes based on common shares																15.6666
Aggregate Effect of Financial Instruments on Common Shares	754
Conversion price of convertible note (in dollars per share)																$ 63.83
Commercial paper borrowings capacity	2,000,000,000
Commercial paper borrowings outstanding	$ 789,200,000	$ 0