Pension Plans and Other Postretirement Benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Defined Benefit Plan Disclosure [Line Items]
Amortization of net actuarial loss			$ (20,718)	$ (18,848)
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6,130	6,875	12,180	14,775
Interest cost on projected benefit obligation	14,939	15,418	30,065	31,018
Expected return on plan assets	(21,802)	(17,581)	(43,677)	(35,256)
Amortization of prior service cost	(1,628)	(1,629)	(3,278)	(3,279)
Amortization of net actuarial loss	10,138	9,183	20,538	18,583
Net periodic benefit cost	7,777	12,266	15,828	25,841
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	171	159	371	334
Interest cost on projected benefit obligation	670	919	1,345	1,869
Expected return on plan assets
Amortization of prior service cost	(329)	10	(679)	10
Amortization of net actuarial loss	80	165	180	265
Net periodic benefit cost	$ 592	$ 1,253	$ 1,217	$ 2,478