Components of Net Periodic Benefit Cost - Net Periodic Benefit Cost (Increase) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Pension benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	$ 0	$ 0	$ 0	$ 0
Interest cost	352	318	704	636
Expected return on plan assets	(894)	(792)	(1,788)	(1,584)
Amortization of prior service cost	0	0	0	0
Amortization of the net loss (gain)	93	338	186	676
Net periodic benefit cost (increase)	(449)	(136)	(898)	(272)
Other post-retirement benefits [Member]
Pension Plans, Postretirement and Other Employee Benefits [Line Items]
Service cost	42	60	84	120
Interest cost	272	245	544	490
Expected return on plan assets	0	0	0	0
Amortization of prior service cost	(1)	(1)	(2)	(2)
Amortization of the net loss (gain)	(51)	4	(102)	8
Net periodic benefit cost (increase)	$ 262	$ 308	$ 524	$ 616