Components of Net Periodic Benefit Cost (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 13,529	$ 15,834	$ 14,788
Interest cost	29,757	26,791	27,363
Expected return on plan assets	(57,550)	(52,480)	(46,967)
Amortization of prior service cost (credit)	1,187	1,786	1,797
Amortization of actuarial losses (gains)	774	12,556	12,168
Net curtailment gain	0	0	(706)
Net periodic benefit (credit) cost	(12,303)	4,487	8,443
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	535	682	830
Interest cost	1,661	1,605	2,051
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	(2,871)	(2,684)	(2,616)
Amortization of actuarial losses (gains)	(328)	(234)	12
Net curtailment gain	0	0	0
Net periodic benefit (credit) cost	$ (1,003)	$ (631)	$ 277