REGULATORY MATTERS Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Schedule of Regulatory Matters
Operating revenues	$ 298,044	$ 275,058	$ 578,177	$ 547,545
Net income	70,726	$ 72,336	134,963	$ 139,468
Deferred income tax liabilities	947,036		$ 947,036		$ 735,426
Transmission rate, applicable period			1 year
Revenue true-up amount reflected in customer bill, period of recognition			2 years
MISO Formula Rate Template Modifications Filing
Schedule of Regulatory Matters
Regulatory liability	6,400		$ 6,400		$ 10,400
Challenges Regarding Bonus Depreciation Election
Schedule of Regulatory Matters
Operating revenues	(3,600)		(9,000)
Net income	(2,200)		(5,400)
Deferred income tax liabilities	$ 152,700		$ 152,700