RETIREMENT BENEFITS AND ASSETS HELD IN TRUST Additional Information (Details) - USD ($)	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2016	Jun. 30, 2016	Jun. 30, 2015
Defined Contribution Plan
Retirement Benefits Disclosure
Plan cost	$ 900,000	$ 900,000		$ 2,500,000	$ 2,400,000
Other Pension Plan
Retirement Benefits Disclosure
Employer contributions				2,800,000
Other Pension Plan | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution			$ 0
Supplemental Retirement Plans
Retirement Benefits Disclosure
Employer contributions				5,200,000
Supplemental Retirement Plans | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution			0
Other Postretirement Benefits Plan
Retirement Benefits Disclosure
Employer contributions				$ 3,700,000
Other Postretirement Benefits Plan | Expectation
Retirement Benefits Disclosure
Expected additional current year contribution			$ 3,700,000