Components of Net Periodic Benefit Expense (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,556	$ 7,603
Interest cost	8,215	6,431
Expected return on plan assets	(8,480)	(5,700)
Amortization of prior service cost	225	276
Amortization of transitional liability	208	120
Recognized actuarial loss	1,733	3,532
Net periodic benefit expense	8,457	12,262
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	672	1,167
Interest cost	1,278	1,234
Expected return on plan assets	0	0
Amortization of prior service cost	(21)	(42)
Amortization of transitional liability	1	2
Recognized actuarial loss	59	457
Net periodic benefit expense	$ 1,989	$ 2,818