Pension Plans and Postretirement Benefits - Components of Net Periodic Benefit Cost (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 Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 9,651	$ 11,542	$ 19,297	$ 23,120
Interest cost	30,361	27,838	60,688	55,626
Expected return on plan assets	(37,389)	(36,611)	(74,766)	(73,281)
Amortization of prior service cost (credit)	635	792	1,269	1,584
Net periodic benefit cost	3,258	3,561	6,488	7,049
Postretirement Benefit Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	216	248	432	496
Interest cost	977	993	1,946	1,898
Expected return on plan assets	(575)	(537)	(1,150)	(1,075)
Amortization of prior service cost (credit)	(40)	(82)	(80)	(73)
Net periodic benefit cost	$ 578	$ 622	$ 1,148	$ 1,246