Capital leases: (Details) (USD $) In Thousands, unless otherwise specified			1 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Jun. 30, 2012 Lease renewal through December 31, 2027 item	Jun. 30, 2012 Lease renewal through June 30, 2031 item	Dec. 31, 2012 Scherer Unit No. 2 item	Dec. 31, 2012 Doyle	Dec. 31, 2000 Doyle item	Dec. 31, 2011 Doyle
Capital leases
Number of purchasers from which entity sold and leased back assets					4
Percentage of undivided ownership interest of purchasers in assets under lease					60.00%
Term of lease extension for which irrevocable notice of renewal is executed			14 years 6 months	18 years
Number of leases for which lease term is extended			3	1
Number of unit generation facilities							5
Agreement period							15 years
Minimum lease payments under the capital leases together with the present value of the net minimum lease payments
2013	$ 35,853				$ 23,406	$ 12,447
2014	27,396				14,949	12,447
2015	33,246				14,949	18,297
2016	14,949				14,949
2017	14,949				14,949
2018-2031	160,178				160,178
Total minimum lease payments	286,571				243,380	43,191
Less: Amount representing interest	(125,322)				(121,258)	(4,064)
Present value of net minimum lease payments	161,249				122,122	39,127
Less: Current portion	(25,306)				(14,924)	(10,382)
Long-term balance	$ 135,943	$ 146,781			$ 107,198	$ 28,745
Interest rate of lessor's debt on which lease payment varies (as a percent)						6.00%
Weighted average interest rate on lease obligation (as a percent)	4.73%	4.85%				5.90%		5.86%