Employee benefit plans - Schedule of Components of Net Periodic Credit (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
Defined Benefit Pension Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	$ 1,110	$ 931	$ 3,445	$ 2,793
Interest cost	4,477	4,752	13,248	14,256
Expected return on plan assets	(12,210)	(13,365)	(36,614)	(39,985)
Amortization of prior service cost	308	308	924	924
Amortization of actuarial losses	4,969	2,624	13,854	7,762
Total credit	(1,346)	(4,750)	(5,143)	(14,250)
Other Postretirement Benefit Plans [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Service cost	37	31	116	95
Interest cost	512	576	1,526	1,723
Amortization of prior service cost	(4,043)	(4,043)	(12,128)	(12,128)
Amortization of actuarial losses	2,091	2,137	5,929	6,412
Total credit	$ (1,403)	$ (1,299)	$ (4,557)	$ (3,898)