Pension and Other Postretirement Plans - Additional Information (Detail) (Employee Group Waiver Plan, USD $) In Millions, unless otherwise specified	3 Months Ended	6 Months Ended
	Jun. 30, 2012	Jun. 30, 2012
Employee Group Waiver Plan
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Employee Group Waiver Plan (EGWP), effective date	January 2013	January 2013
Reduction in postretirement health care plan liability		$ 26.7
Increase in net actuarial gain	$ 0.9	$ 1.5