Components of Net Periodic Benefit Cost (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
Other Postretirement Benefit Plans, Defined Benefit [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 191	$ 228	$ 392	$ 455
Interest cost	443	571	917	1,140
Amortization of prior service cost	(669)	(654)	(1,339)	(1,308)
Amortization of actuarial losses	49	53	104	142
Net periodic benefit cost	14	198	74	429
Pension Benefit Plans
Defined Benefit Plan Disclosure [Line Items]
Service cost	3,947	3,676	8,056	7,399
Interest cost	6,624	6,920	13,272	13,829
Expected return on plan assets	(13,156)	(11,801)	(26,269)	(22,314)
Amortization of prior service cost	421	477	849	953
Amortization of actuarial losses	2,898	3,088	5,885	6,176
Net periodic benefit cost	$ 734	$ 2,360	$ 1,793	$ 6,043