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Employee benefit plans - Summary of Key Actuarial Assumptions Used to Determine Net Benefit Obligations (Detail) (OPEB [Member])	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011
OPEB [Member]
Healthcare inflation:
Initial rate	7.00%	8.00%
Ultimate rate	5.00%	5.00%
Year of ultimate rate achievement	2018	2018
Discount rate	3.47%	3.93%

The assumed health care cost trend rate for the next year used to measure the expected cost of benefits covered by the plan (gross eligible charges). This is based upon the annual rate of change in the cost of health care benefits currently provided by the postretirement benefit plan, due to factors other than changes in the composition of the plan population by age and dependency status.

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