Quarterly report pursuant to Section 13 or 15(d)

Fair Value Of Financial Instruments (Fair Value, Assets Measured On Recurring Basis, Unobservable Input Reconciliation) (Detail)

v2.4.0.6
Fair Value Of Financial Instruments (Fair Value, Assets Measured On Recurring Basis, Unobservable Input Reconciliation) (Detail) (USD $)
In Millions, unless otherwise specified
3 Months Ended 9 Months Ended
Sep. 30, 2012
Sep. 30, 2011
Jun. 30, 2011
Sep. 30, 2012
Sep. 30, 2011
Sep. 30, 2012
Fair Value Assets And Liabilities Measured On Recurring Basis Unobservable Input Reconciliation [Roll Forward]            
Balance $ 83.9 $ 64.0   $ 157.9 $ 45.6  
Total gains            
Included in earnings 24.9 53.8   129.6 144.9  
Included in other comprehensive income 0 0   0 0  
Settlements (52.9) (50.0)   (231.6) (102.7)  
Transfers into Level 3 0 0   0 0  
Transfers out of Level 3 0 0 (20.0) 0 (20.0)  
Balance 55.9 67.8 64.0 55.9 67.8  
Total gains for the period included in earnings attributable to the change in unrealized gains on assets still held at the reporting date 24.9 53.8   129.6 144.9  
Asset retirement obligation at beginning of period       $ 194.9 $ 162.1 $ 208.3