SEGMENT INFORMATION (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues	$ 136,758	$ 169,714	$ 134,557	$ 164,544	$ 151,363	$ 171,512	$ 149,782	$ 179,673	$ 605,573	$ 652,330	$ 594,330
Depreciation and amortization									80,550	73,185	69,306
Federal and state income taxes									33,812	39,220	37,492
Operating income	19,757	35,783	16,047	24,713	19,300	31,709	19,502	29,488	96,301	99,999	99,663
Interest income									145	51	566
Interest expense									45,103	41,740	41,479
Income from AFUDC (debt and equity)									7,695	9,917	5,940
Income from continuing operations									56,597	67,103	63,445
Net income	9,905	25,285	6,770	14,637	11,112	23,892	11,194	20,905	56,597	67,103	63,445
Capital Expenditures									176,524	222,853	160,208
Total assets	2,455,303				2,371,056				2,455,303	2,371,056
Goodwill	39,492				39,492				39,492	39,492
SPP IM
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues	2,900	$ 4,000	$ 3,400	$ 4,700	7,500	$ 11,500	$ 16,500	$ 6,200
Electric
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues									555,085	592,491	536,413
Depreciation and amortization									74,732	67,534	63,659
Federal and state income taxes									31,123	35,737	34,478
Operating income									88,124	90,488	90,984
Interest income									133	37	537
Interest expense									41,307	37,911	37,683
Income from AFUDC (debt and equity)									7,681	9,833	5,910
Income from continuing operations									52,240	61,467	58,603
Capital Expenditures									169,111	212,866	153,401
Total assets	2,339,850				2,252,339				2,339,850	2,252,339
Electric | SPP IM
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues									15,000	41,900
Gas
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues									41,702	51,842	50,041
Depreciation and amortization									3,923	3,760	3,709
Federal and state income taxes									800	1,840	1,484
Operating income									5,153	6,775	6,194
Interest income									36	25	115
Interest expense									3,867	3,861	3,890
Income from AFUDC (debt and equity)									14	84	30
Income from continuing operations									1,287	2,965	2,355
Capital Expenditures									5,190	7,836	4,419
Total assets	127,871				130,856				127,871	130,856
Goodwill	39,492				39,492				39,492	39,492
Other
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues									10,165	9,302	9,147
Depreciation and amortization									1,895	1,891	1,938
Federal and state income taxes									1,889	1,643	1,530
Operating income									3,024	2,736	2,485
Interest income									47	21	8
Income from continuing operations									3,070	2,671	2,487
Capital Expenditures									2,223	2,151	2,388
Total assets	38,300				34,655				38,300	34,655
Eliminations
Statement of Income Information, balance sheet information and capital expenditures of business segments
Operating Revenues									(1,379)	(1,305)	(1,271)
Interest income									(71)	(32)	(94)
Interest expense									(71)	(32)	$ (94)
Total assets	$ (50,718)				$ (46,794)				$ (50,718)	$ (46,794)