Income taxes (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Income taxes
Computed income taxes at Canadian statutory rate	$ 16,300,000
Canadian enacted statutory rate (as a percent)	25.00%
Income tax reconciliation, adjustment for taxable losses in higher state and local tax jurisdictions	(13,900,000)
Components of income tax expenses (benefit)
Current income tax expense (benefit)	1,385,000	(488,000)
Deferred tax expense (benefit)	(17,676,000)	2,011,000
Total income tax expense (benefit)	(16,291,000)	1,523,000
Deferred tax assets:
Valuations allowance	$ 97,400,000