Pension plans and other postretirement benefits - Net Periodic Benefit Cost for Defined Benefit Plans (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Other postretirement benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 200	$ 175
Interest cost on projected benefit obligation	675	950
Expected return on plan assets
Amortization of prior service credit	(350)
Amortization of net actuarial loss	100	100
Net periodic benefit cost	625	1,225
Pension benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6,050	7,900
Interest cost on projected benefit obligation	15,126	15,600
Expected return on plan assets	(21,875)	(17,675)
Amortization of prior service credit	(1,650)	(1,650)
Amortization of net actuarial loss	10,400	9,400
Net periodic benefit cost	$ 8,051	$ 13,575