Pension plans and other postretirement benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Defined Benefit Plan Disclosure [Line Items]
Recognized net actuarial loss (gain)	$ 37,916	$ 20,566
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	29,549	28,828	19,670
Interest cost on benefit obligation	62,037	58,545	47,905
Expected return on plan assets	(70,511)	(60,700)	(50,844)
Amortization of prior service cost	(6,559)	(6,559)	(6,559)
Recognized net actuarial loss (gain)	37,386	20,530	13,551
Net periodic benefit costs	51,902	40,644	23,723
Other postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	668	535	383
Interest cost on benefit obligation	3,737	3,761	3,130
Amortization of prior service cost	21	107	176
Recognized net actuarial loss (gain)	530	36	(9)
Net periodic benefit costs	$ 4,956	$ 4,439	$ 3,680