Long-Term Debt and Capital Leases (Tables)

12 Months Ended

Dec. 31, 2016

Long-term Debt and Capital Lease Obligations [Abstract]

Schedule of Debt and Capital Leases

Long-term debt and capital leases consisted of the following (in thousands):

		December 31,
	Due	2016			2015
Unsecured Debt:
Unsecured Revolving Line of Credit	2021	$	—		$	—
Secured Debt:
Mortgage bonds—
South Dakota—6.05%	2018	—			55,000
South Dakota—5.01%	2025	64,000			64,000
South Dakota—4.15%	2042	30,000			30,000
South Dakota—4.30%	2052	20,000			20,000
South Dakota—4.85%	2043	50,000			50,000
South Dakota—4.22%	2044	30,000			30,000
South Dakota—4.26%	2040	70,000			70,000
South Dakota—2.80%	2026	60,000			—
South Dakota—2.66%	2026	45,000			—
Montana—6.34%	2019	250,000			250,000
Montana—5.71%	2039	55,000			55,000
Montana—5.01%	2025	161,000			161,000
Montana—4.15%	2042	60,000			60,000
Montana—4.30%	2052	40,000			40,000
Montana—4.85%	2043	15,000			15,000
Montana—3.99%	2028	35,000			35,000
Montana—4.176%	2044	450,000			450,000
Montana—3.11%	2025	75,000			75,000
Montana—4.11%	2045	125,000			125,000
Pollution control obligations—
Montana—4.65%	2023	—			170,205
Montana—2.00%	2023	144,660			—
Other Long Term Debt:
New Market Tax Credit Financing—1.146%	2046	26,977			26,977
Discount on Notes and Bonds and Debt Issuance Costs, Net	—	(13,299		)	(13,999		)
		$	1,793,338		$	1,768,183
Less current maturities		—			—
		$	1,793,338		$	1,768,183
Capital Leases:
Total Capital Leases	Various	$	26,325		$	28,162
Less current maturities		(1,979		)	(1,837		)
		$	24,346		$	26,325