Summary of significant accounting policies: (Details 3) (USD $)	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Asset retirement obligations
Estimated annual increase in regulatory asset for asset retirement obligations	$ 200,000
Asset Retirement Obligations
Balance at the beginning of the period	298,758,000	280,496,000
Liabilities incurred	1,632,000	423,000
Liabilities settled	(1,117,000)	(410,000)
Accretion	19,554,000	18,249,000	17,131,000
Change in Cash Flow Estimate	62,535,000
Balance at the end of the period	$ 381,362,000	$ 298,758,000	$ 280,496,000