Employee Benefits - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
	Sep. 30, 2012	Sep. 30, 2012	Sep. 30, 2011
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement contributions		$ 142.6	$ 44.9
Pension and postretirement expected contributions for next year	40	40
Recognized expense	7.2	23.3
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement contributions		135.8
Pension Benefits [Member] | Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension expected contributions- current year	143	143
Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension and other postretirement contributions		6.8
Postretirement Benefits [Member] | Subsequent Event [Member]
Defined Benefit Plan Disclosure [Line Items]
Pension expected contributions- current year	$ 9	$ 9