REGULATORY MATTERS Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2016	Mar. 31, 2015	Dec. 31, 2015
Schedule of Regulatory Matters [Line Items]
Operating revenues	$ 280,133	$ 272,487
Net income	64,237	$ 67,132
Deferred income tax liabilities	$ 898,858		$ 735,426
Transmission rate, applicable period	1 year
Revenue true-up amount reflected in customer bill, period of recognition	2 years
Challenges Regarding Bonus Depreciation Election | Impact from Bonus Depreciation Election
Schedule of Regulatory Matters [Line Items]
Operating revenues	$ (5,400)
Net income	(3,200)
Deferred income tax liabilities	138,700
MISO Formula Rate Template Modifications Filing
Schedule of Regulatory Matters [Line Items]
Regulatory liability	$ 8,400		$ 10,400