Pension and Post-Retirement Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension Benefits
Components of net periodic expense (credit):
Interest cost	$ 1,471	$ 1,491	$ 4,414	$ 4,473
Expected return on plan assets	(3,314)	(3,133)	(9,943)	(9,399)
Amortization of unrecognized loss	2,434	1,190	7,303	3,569
Net periodic expense (credit)	591	(452)	1,774	(1,357)
Post-Retirement Benefits
Components of net periodic expense (credit):
Interest cost	160	180	481	540
Service cost	2	1	5	4
Unrecognized past service liability	(62)	(62)	(187)	(187)
Amortization of unrecognized loss	126	103	379	308
Net periodic expense (credit)	$ 226	$ 222	$ 678	$ 665