Long-Term Debt and Capital Leases Schedule of Debt (Details) (USD $) In Thousands, unless otherwise specified			12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended		12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2012 Unsecured Debt Unsecured Revolving Line Of Credit	Dec. 31, 2011 Unsecured Debt Unsecured Revolving Line Of Credit	Dec. 31, 2012 Secured Debt South Dakota, 6.05%, Due 2018	Dec. 31, 2011 Secured Debt South Dakota, 6.05%, Due 2018	Dec. 31, 2012 Secured Debt South Dakota, 5.01%, Due 2025	Dec. 31, 2011 Secured Debt South Dakota, 5.01%, Due 2025	Dec. 31, 2012 Secured Debt South Dakota, 4.15%, Due 2042	Dec. 31, 2011 Secured Debt South Dakota, 4.15%, Due 2042	Dec. 31, 2012 Secured Debt South Dakota, 4.30%, Due 2052	Dec. 31, 2011 Secured Debt South Dakota, 4.30%, Due 2052	Dec. 31, 2012 Secured Debt Montana, 6.04%, Due 2016	Dec. 31, 2011 Secured Debt Montana, 6.04%, Due 2016	Dec. 31, 2012 Secured Debt Montana, 6.34%, Due 2019	Dec. 31, 2011 Secured Debt Montana, 6.34%, Due 2019	Dec. 31, 2012 Secured Debt Montana, 5.71%, Due 2039	Dec. 31, 2011 Secured Debt Montana, 5.71%, Due 2039	Dec. 31, 2012 Secured Debt Montana, 5.01%, Due 2025	Dec. 31, 2011 Secured Debt Montana, 5.01%, Due 2025	Dec. 31, 2012 Secured Debt Montana, 4.15%, Due 2042	Dec. 31, 2011 Secured Debt Montana, 4.15%, Due 2042	Dec. 31, 2012 Secured Debt Montana, 4.30%, Due 2052	Dec. 31, 2011 Secured Debt Montana, 4.30%, Due 2052	Dec. 31, 2012 Secured Debt Montana, 4.65%, Due 2023	Dec. 31, 2011 Secured Debt Montana, 4.65%, Due 2023	Dec. 31, 2011 Secured Debt Montana Natural Gas Transition Bonds, 6.20%, Due 2012	Dec. 31, 2012 Secured Debt Montana Natural Gas Transition Bonds, 6.20%, Due 2012	Dec. 31, 2012 Unamortized Discount Premium, Net Discount on Notes and Bonds	Dec. 31, 2011 Unamortized Discount Premium, Net Discount on Notes and Bonds
Debt Instrument [Line Items]
Long-term debt	$ 1,055,074	$ 908,841	$ 0	$ 0	$ 55,000	$ 55,000	$ 64,000	$ 64,000	$ 30,000	$ 0	$ 20,000	$ 0	$ 150,000	$ 150,000	$ 250,000	$ 250,000	$ 55,000	$ 55,000	$ 161,000	$ 161,000	$ 60,000	$ 0	$ 40,000	$ 0	$ 170,205	$ 170,205	$ 3,792	$ 0	$ (131)	$ (156)
Less current maturities	0	(3,792)
Long-term debt, excluding current maturities	1,055,074	905,049
Interest rate, stated percentage					6.05%		5.01%		4.15%		4.30%		6.04%		6.34%		5.71%		5.01%		4.15%		4.30%		4.65%		6.20%
Maturity date			Jun. 30, 2016		May 01, 2018		May 01, 2025		Aug. 10, 2042		Aug. 10, 2052		Sep. 01, 2016		Apr. 01, 2019		Oct. 15, 2039		May 01, 2025		Aug. 10, 2042		Aug. 10, 2052		Aug. 01, 2023		Mar. 15, 2012
Capital Lease
Total Capital Leases	33,174	34,288
Less current maturities	(1,612)	(1,370)
Capital lease obligations, noncurrent	$ 31,562	$ 32,918