Summary of significant accounting policies: (Details 3) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Asset Retirement Obligations
Balance at the beginning of the period	$ 381,362	$ 298,758
Liabilities incurred		1,632
Liabilities settled	(1,392)	(1,117)
Accretion	22,900	19,554	18,249
Change in Cash Flow Estimates	5,180	62,535
Balance at the end of the period	$ 408,050	$ 381,362	$ 298,758