Pension plans and other postretirement benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,387	$ 8,557	$ 22,162	$ 20,270
Interest cost on projected benefit obligation	15,509	16,155	46,527	42,391
Expected return on plan assets	(17,627)	(16,718)	(52,883)	(43,981)
Amortization of prior service cost	(1,640)	(1,640)	(4,919)	(4,919)
Amortization of net actuarial loss	9,291	5,132	27,874	15,397
Net periodic benefit cost	12,920	11,486	38,761	29,158
Other postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	167	148	501	388
Interest cost on projected benefit obligation	934	1,038	2,803	2,722
Amortization of prior service cost	5	27	15	81
Amortization of net actuarial loss	133	9	398	27
Net periodic benefit cost	$ 1,239	$ 1,222	$ 3,717	$ 3,218