Subsequent events (Details) (USD $)	Sep. 30, 2014	Oct. 31, 2014 Unsecured Revolving Credit Facility expires in 2018 [Member] Subsequent Event [Member]	Sep. 30, 2014 Line of Credit [Member] Export Development Canada and secured equipment credit facility [Member]
Subsequent Event [Line Items]
Ratio Of Total Indebtedness To Adjusted EBITDA through the quarter ended December 31, 2015		6.5
Ratio Of Total Indebtedness To Adjusted EBITDA through the quarter ended December 31, 2016		6.25
Ratio Of Total Indebtedness To Adjusted EBITDA	2.5
Required Ratio Of Total Indebtedness To Adjusted EBITDA for each fiscal quarter ending thereafter through expiration of the revolving credit facility		6.0
Minimum required cash remaining on hand for operations as part of covenants		$ 2,000,000,000
Lines of Credit Facility, Amount Outstanding			$ 1,100,000,000