Schedule II, Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Allowance for Bad Debts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		$ 57,719	$ 53,107	$ 51,394
Charged to costs and expenses		36,074	42,765	37,627
Charged to other accounts	[1]	11,784	14,067	14,469
Deductions from reserves	[2]	53,689	52,220	50,383
Balance at Dec. 31		51,888	57,719	53,107
NOL and Tax Credit Valuation Allowances
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		3,402	3,263	3,314
Charged to costs and expenses		2,064	139	0
Charged to other accounts	[1]	24,784	0	0
Deductions from reserves	[2]	2,571	0	51
Balance at Dec. 31		$ 27,679	$ 3,402	$ 3,263

[1] Recovery of amounts previously written off as related to allowance for bad debts. Accrual of valuation allowance for North Dakota ITC, offset to regulatory liability.

[2] Deductions related to allowance for bad debts relates primarily to write-offs. Reductions to valuation allowances for North Dakota ITC carryforwards primarily due to a consolidated adjustment to the regulatory liability accrual referenced above. Reductions to valuation allowances for NOL carryforwards primarily due to changes in forecasted taxable income.