Schedule II, Valuation and Qualifying Accounts (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Allowance for Bad Debts
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		$ 51,888	$ 57,719	$ 53,107
Charged to costs and expenses		38,960	36,074	42,765
Charged to other accounts	[1]	10,570	11,784	14,067
Deductions from reserves	[2]	50,595	53,689	52,220
Balance at Dec. 31		50,823	51,888	57,719
NOL and Tax Credit Valuation Allowances
Movement in Valuation Allowances and Reserves [Roll Forward]
Balance at Jan. 1		27,679	3,402	3,263
Charged to costs and expenses		3,175	2,064	139
Charged to other accounts	[1]	34,637	24,784	0
Deductions from reserves	[2]	7,976	2,571	0
Balance at Dec. 31		$ 57,515	$ 27,679	$ 3,402

[1] Accrual of valuation allowance for North Dakota ITC, offset to regulatory liability.

[2] 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.