Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Pension
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 250	$ 224	$ 504	$ 481
Interest cost	645	681	1,291	1,361
Expected return on plan assets	(623)	(570)	(1,244)	(1,139)
Amortization of prior service cost	74	74	148	148
Amortization of net loss	445	322	884	626
Net periodic benefit cost	791	731	1,583	1,477
Post-Retirement Healthcare
Defined Benefit Plan Disclosure [Line Items]
Service cost	2	4	5	8
Interest cost	185	209	370	419
Amortization of transition obligation		28		57
Amortization of prior service cost	21	47	42	93
Amortization of net loss	44	28	88	56
Net periodic benefit cost	$ 252	$ 316	$ 505	$ 633