Basis of presentation and significant accounting policies - Supplemental cash flow information (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Accounting Policies [Abstract]
Cash paid for interest, net of $0 and $212 of capitalized interest, respectively			$ 46,140	$ 48,348
Capitalized interest			0	212
Change in accrued capital expenditures			13,346	(33,892)
Capitalized asset retirement cost			1,591	1,262
Capitalized stock-based compensation	1,249	0	2,167	0
Restricted deposits on pending sale			$ 0	$ 44,000