Debt: (Details) (USD $)	12 Months Ended				12 Months Ended											0 Months Ended
	Dec. 31, 2012 item	Dec. 31, 2011	Dec. 31, 2012 Long-term Debt and Capital Leases	Dec. 31, 2012 Long-term Debt	Dec. 31, 2012 4.20% First Mortgage Bonds, Series 2012	Dec. 31, 2012 FFB	Dec. 31, 2012 CFC	Dec. 31, 2012 CoBank	Dec. 31, 2012 PCBs	Dec. 31, 2012 CREBs	Dec. 31, 2012 Term Loans	Dec. 31, 2012 Capital Leases	Dec. 31, 2012 Committed credit arrangements item	Dec. 31, 2012 Letters of credit	Dec. 31, 2012 Commercial paper	Mar. 02, 2013 Pollution control revenue bonds Subsequent events
Debt
Number of commercial banks to whom unsecured notes is payable	3
Debt issued					$ 250,000,000	$ 107,390,000
Refund of pollution control revenue bonds																212,760,000
Maturities for long-term debt and capital lease obligations
2013			168,393,000	143,087,000		103,407,000	767,000	551,000	37,352,000	1,010,000		25,306,000
2014			421,664,000	407,452,000		107,663,000	806,000	621,000	37,352,000	1,010,000	260,000,000	14,212,000
2015			306,939,000	285,664,000		112,206,000	848,000	698,000	170,902,000	1,010,000		21,275,000
2016			123,200,000	119,245,000		116,558,000	891,000	786,000		1,010,000		3,955,000
2017			120,880,000	116,476,000		113,644,000	937,000	885,000		1,010,000		4,404,000
Weighted average interest rate for long-term debt and capital leases	4.73%	4.85%
Maximum borrowing capacity													1,925,000,000	835,000,000
Available borrowing capacity														583,000,000
Number of separate facilities													5
Line of credit, amount outstanding															569,000,000
Letters of credit, amount outstanding														$ 253,000,000
Weighted average interest rate on short-term borrowings (as a percent)	0.44%	0.26%