STOCKHOLDERS EQUITY Additional Information (Details)						6 Months Ended	12 Months Ended
	Dec. 31, 2015 USD ($) $ / shares shares	Nov. 05, 2015 shares	Oct. 01, 2015 shares	Sep. 30, 2015 USD ($) $ / shares	Jun. 19, 2014 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2015 USD ($) $ / shares shares	Dec. 31, 2014 USD ($) shares	Dec. 31, 2013 USD ($) shares	Apr. 30, 2014 USD ($)	Dec. 31, 2012 shares
Class of Stock [Line Items]
Common stock, shares authorized | shares	300,000,000					300,000,000	300,000,000	300,000,000
Common stock, par value | $ / shares	$ 0						$ 0
Preferred stock, authorized shares | shares	10,000,000						10,000,000
Preferred stock, par value | $ / shares	$ 0						$ 0
Common stock, shares outstanding | shares	152,699,077					155,140,967	152,699,077	155,140,967
Preferred stock, outstanding shares | shares	0						0
Common stock, shareholders of record	838						838
Stock repurchase program, authorized amount										$ 250,000,000
Return of unused advance for forward contract of accelerated share repurchase program							$ 0	$ 20,000,000	$ 0
Shares issued of preferred stock | shares	0						0
FERC approved capital structure, equity percentage	60.00%						60.00%
FERC approved capital structure, debt percentage	40.00%						40.00%
Common Stock
Class of Stock [Line Items]
Common stock, shares outstanding | shares	152,699,077					155,140,967	152,699,077	155,140,967	157,500,795		156,745,542
Repurchase and retirement of common stock, shares | shares							4,201,847	3,673,226	163,320
Repurchase and retirement of common stock							$ 137,081,000	$ 134,284,000	$ 4,885,000
2014 ASR Program
Class of Stock [Line Items]
Accelerated share repurchase agreement, maximum amount					$ 150,000,000
Accelerated share repurchase agreement, minimum amount					130,000,000
Accelerated share repurchase agreement, cash payment					150,000,000
Accelerated share repurchase agreement, fair market value of initial delivery					$ 104,000,000
Accelerated share repurchases, initial price paid per share | $ / shares					$ 35.80
Accelerated share repurchase agreement, settlement amount					$ 130,000,000
Return of unused advance for forward contract of accelerated share repurchase program								$ 20,000,000
2015 ASR Program
Class of Stock [Line Items]
Accelerated share repurchase agreement, cash payment				$ 115,000,000
Accelerated share repurchase agreement, fair market value of initial delivery				$ 92,000,000
Accelerated share repurchases, initial price paid per share | $ / shares				$ 33.34
Accelerated share repurchase agreement amount				$ 115,000,000.0
Initial Share Delivery | 2014 ASR Program
Class of Stock [Line Items]
Accelerated share repurchase agreement, share delivery and retirement | shares					2,900,000
Initial Share Delivery | 2015 ASR Program
Class of Stock [Line Items]
Accelerated share repurchase agreement, share delivery and retirement | shares			2,800,000
Settlement Share Delivery | 2014 ASR Program
Class of Stock [Line Items]
Accelerated share repurchase agreement, share delivery and retirement | shares					700,000
Settlement Share Delivery | 2015 ASR Program
Class of Stock [Line Items]
Accelerated share repurchase agreement, share delivery and retirement | shares		800,000
ASR Program | 2014 ASR Program
Class of Stock [Line Items]
Repurchase and retirement of common stock						$ 130,000,000
ASR Program | 2015 ASR Program
Class of Stock [Line Items]
Repurchase and retirement of common stock	$ 115,000,000