Business Segments Information Relating to Segments Statement of Income (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended											12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012		Sep. 30, 2012		Jun. 30, 2012		Mar. 31, 2012	Dec. 31, 2013		Dec. 31, 2012		Dec. 31, 2011
Segment Reporting Information
Revenue	$ 355,448	$ 259,907	$ 279,826	$ 380,671	$ 318,862		$ 246,808		$ 242,363		$ 365,851	$ 1,275,852		$ 1,173,884		$ 1,272,188
Fuel, purchased power and cost of natural gas sold												492,147		407,066		574,989
Operations and maintenance												386,936		370,736		375,369
Gain on sale of operating assets					1,800		27,000					0		29,129	[1]	0
Depreciation, depletion and amortization												141,217		154,632		135,591
Impairment of long-lived assets									27,000			0		26,868	[2]	0
Operating income	71,103	55,566	49,037	79,846	75,262	[3]	77,810	[3]	20,591	[3]	70,048	255,552		243,711		186,239
Interest expense												(111,788)	[4]	(113,610)	[5]	(91,383)
Unrealized gain (loss) on interest rate swaps, net												30,169		1,882		(42,010)
Interest income												1,723		1,957		2,017
Other income (expense), net												1,798		2,965		3,726
Income tax benefit (expense)												(61,608)		(48,400)		(18,224)
Income (loss) from continuing operations												115,846		88,505		40,365
Electric Utilities [Member]
Segment Reporting Information
Revenue												651,445		610,732		600,935
Fuel, purchased power and cost of natural gas sold												294,048		273,474		310,352
Operations and maintenance												159,961		146,527		142,815
Gain on sale of operating assets												0		0		768	[6]
Depreciation, depletion and amortization												77,704		75,244		52,475
Operating income												133,595		131,721		109,457
Interest expense												(61,537)		(59,194)		(53,770)
Unrealized gain (loss) on interest rate swaps, net												0		0		0
Interest income												5,277		8,153		14,794
Other income (expense), net												633		1,182		481
Income tax benefit (expense)												(25,834)		(30,264)		(23,271)
Income (loss) from continuing operations												52,134		51,598		47,691
Gas Utilities [Member]
Segment Reporting Information
Revenue												539,689		454,081		554,584
Fuel, purchased power and cost of natural gas sold												310,463		245,349		331,961
Operations and maintenance												126,073		117,390		121,980
Gain on sale of operating assets												0		0		0
Depreciation, depletion and amortization												26,381		25,163		24,307
Operating income												76,772		66,179		76,336
Interest expense												(25,234)		(26,746)		(31,621)
Unrealized gain (loss) on interest rate swaps, net												0		0		0
Interest income												976		2,765		5,645
Other income (expense), net												(60)		105		217
Income tax benefit (expense)												(19,747)		(14,313)		(16,408)
Income (loss) from continuing operations												32,707		27,990		34,169
Power Generation [Member]
Segment Reporting Information
Revenue												4,648		4,189		4,059
Fuel, purchased power and cost of natural gas sold												0		0		0
Operations and maintenance												30,186		29,991		16,538
Gain on sale of operating assets												0		0		0
Depreciation, depletion and amortization												5,091		4,599		4,199
Operating income												47,760		44,799		10,935
Interest expense												(21,178)	[4]	(15,452)		(8,903)
Unrealized gain (loss) on interest rate swaps, net												0		0		0
Interest income												785		695		1,529
Other income (expense), net												1		7		1,094
Income tax benefit (expense)												(11,080)		(8,721)		(1,644)
Income (loss) from continuing operations												16,288		21,328		3,011
Coal Mining [Member]
Segment Reporting Information
Revenue												25,186		25,810		32,802
Fuel, purchased power and cost of natural gas sold												0		0		0
Operations and maintenance												39,519		42,553		56,617
Gain on sale of operating assets												0		0		0
Depreciation, depletion and amortization												11,523		13,060		18,670
Operating income												5,586		2,165		(8,395)
Interest expense												(641)		(238)		(9)
Unrealized gain (loss) on interest rate swaps, net												0		0		0
Interest income												10		1,168		3,897
Other income (expense), net												2,304		2,616		2,192
Income tax benefit (expense)												(932)		(85)		1,891
Income (loss) from continuing operations												6,327		5,626		(424)
Oil and Gas [Member]
Segment Reporting Information
Revenue												54,884		79,072		79,808
Fuel, purchased power and cost of natural gas sold												0		0		0
Operations and maintenance												40,365		43,267		41,380
Gain on sale of operating assets												0		29,129	[1]	0
Depreciation, depletion and amortization												21,770		38,494		35,690
Impairment of long-lived assets														26,868	[2]
Operating income												(7,251)		(428)		2,738
Interest expense												(2,253)		(4,539)		(5,896)
Unrealized gain (loss) on interest rate swaps, net												0		0		0
Interest income												1,639		604		2
Other income (expense), net												108		207		(216)
Income tax benefit (expense)												3,545		1,927		1,651
Income (loss) from continuing operations												(4,212)		(2,229)		(1,721)
Intercompany Eliminations [Member]
Segment Reporting Information
Revenue												(344,314)		(319,855)		(267,349)
Fuel, purchased power and cost of natural gas sold												(112,489)		(111,757)		(67,421)
Operations and maintenance												(211,977)		(188,051)		(174,908)
Gain on sale of operating assets												0		0		(767)	[6]
Depreciation, depletion and amortization												(12,876)		(12,864)		(10,955)
Operating income												(6,972)		(7,183)		(14,832)
Interest expense												84,250		85,209		102,130
Unrealized gain (loss) on interest rate swaps, net												0		0		0
Interest income												(76,724)		(76,123)		(88,149)
Other income (expense), net												(42,641)		(49,921)		(46,552)
Income tax benefit (expense)												218		(131)		268
Income (loss) from continuing operations												(41,869)		(48,149)		(47,135)
Intercompany Eliminations [Member] | Electric Utilities [Member]
Segment Reporting Information
Revenue												13,863		16,234		13,396
Intercompany Eliminations [Member] | Gas Utilities [Member]
Segment Reporting Information
Revenue												0		0		0
Intercompany Eliminations [Member] | Power Generation [Member]
Segment Reporting Information
Revenue												78,389		75,200		27,613
Intercompany Eliminations [Member] | Coal Mining [Member]
Segment Reporting Information
Revenue												31,442		31,968		34,090
Intercompany Eliminations [Member] | Oil and Gas [Member]
Segment Reporting Information
Revenue												0		0		0
Operating Segments [Member] | Electric Utilities [Member]
Segment Reporting Information
Revenue												665,308		626,966		614,331
Operating Segments [Member] | Gas Utilities [Member]
Segment Reporting Information
Revenue												539,689		454,081		554,584
Operating Segments [Member] | Power Generation [Member]
Segment Reporting Information
Revenue												83,037		79,389		31,672
Operating Segments [Member] | Coal Mining [Member]
Segment Reporting Information
Revenue												56,628		57,778		66,892
Operating Segments [Member] | Oil and Gas [Member]
Segment Reporting Information
Revenue												54,884		79,072		79,808
Corporate, Non-Segment [Member]
Segment Reporting Information
Revenue												220,620		196,453		192,250
Fuel, purchased power and cost of natural gas sold												125		0		97
Operations and maintenance												202,809		179,059		170,947
Gain on sale of operating assets												0		0		(1)
Depreciation, depletion and amortization												11,624		10,936		11,205
Operating income												6,062		6,458		10,000
Interest expense												(85,195)	[4]	(92,650)	[5]	(93,314)
Unrealized gain (loss) on interest rate swaps, net												30,169		1,882		(42,010)
Interest income												69,760		64,695		64,299
Other income (expense), net												41,453		48,769		46,510
Income tax benefit (expense)												(7,778)		3,187		19,289
Income (loss) from continuing operations												54,471		32,341		4,774
Consolidation, Eliminations [Member]
Segment Reporting Information
Revenue												$ 0		$ 0		$ 0

[1] Oil and Gas includes gain on sale of the Williston Basin assets (see Note 21).

[2] Oil and Gas includes a ceiling test impairment (see Note 12).

[3] Second quarter includes a pre-tax ceiling test impairment loss of $27 million and the third and fourth quarters include a pre-tax gain on sale of the Williston Basin assets of $27 million and $1.8 million, respectively.

[4] Power Generation includes costs associated with interest rate swaps settled and write-off of deferred financing costs upon repayment of Black Hills Wyoming Project Financing and Corporate includes a the write-off of deferred financing costs and a make-whole provision from early repayment of long-term debt (see Note 5).

[5] Corporate includes a make-whole provision from early repayment of long-term debt (see Note 5).

[6] Electric Utilities includes gain on sale of assets to a related party which was eliminated in consolidation.