Pension Benefit and Other Postretirement Benefit Plans - Additional Information (Detail) (USD $)	3 Months Ended			9 Months Ended
	Sep. 30, 2012	Mar. 31, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined benefit pension plans, voluntary contributions	$ 6,300,000
President and Chief Executive Officer
Pension and Other Postretirement Benefits Disclosure [Line Items]
Defined benefit plan, one-time cash payment	6,300,000
Retirement Health and Life Insurance Benefits
Pension and Other Postretirement Benefits Disclosure [Line Items]
Special termination benefit		1,600,000		1,593,000
Settlement charge
Pension Benefits
Pension and Other Postretirement Benefits Disclosure [Line Items]
Special termination benefit
Settlement charge	2,023,000			2,023,000
Defined benefit pension plans, voluntary contributions	$ 0		$ 0	$ 16,000,000	$ 0