Capital leases: (Details) (USD $) In Thousands, unless otherwise specified			1 Months Ended		12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Jun. 30, 2012 Lease renewal through December 31, 2027 item	Jun. 30, 2012 Lease renewal through June 30, 2031 item	Dec. 31, 2013 Scherer Unit No. 2 item	Dec. 31, 2013 Doyle	Dec. 31, 2000 Doyle item	Dec. 31, 2012 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
2014	$ 27,396				$ 14,949	$ 12,447
2015	33,247				14,949	18,298
2016	14,949				14,949
2017	14,949				14,949
2018	14,949				14,949
2019-2031	145,230				145,230
Total minimum lease payments	250,720				219,975	30,745
Less: Amount representing interest	(110,508)				(108,508)	(2,000)
Present value of net minimum lease payments	140,212				111,467	28,745
Less: Current portion	(18,481)				(7,459)	(11,022)
Long-term balance	$ 121,731	$ 135,943			$ 104,008	$ 17,723
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)	4.56%	4.73%				5.94%		5.90%