Accumulated other comprehensive income (loss) - Schedule of Accumulated Other Comprehensive Income Comprises Changes in Equity (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Accumulated Other Comprehensive Losses [Line Items]
Balance at beginning of year	$ (177,042)	$ (182,093)
Other comprehensive income	1,367	1,585
Balance at March 31	(175,675)	(180,508)
Defined Benefit Pension Plans [Member]
Accumulated Other Comprehensive Losses [Line Items]
Balance at beginning of year	(179,731)	(192,552)
Amortization of prior service cost	186	186
Amortization of net actuarial losses	2,495	2,682
Balance at March 31	(177,050)	(189,684)
OPEB [Member]
Accumulated Other Comprehensive Losses [Line Items]
Balance at beginning of year	2,689	10,459
Amortization of prior service cost	(2,442)	(2,442)
Amortization of net actuarial losses	1,128	1,159
Balance at March 31	$ 1,375	$ 9,176