LONG-TERM DEBT - Other Information (Details)	3 Months Ended
	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)
Debt
Long-term debt	$ 3,303,207,000	$ 3,144,906,000
Unsecured debt
Number of unsecured debt instruments	1
Unsecured notes	$ 25,300,000
Credit facility information
Outstanding borrowings	3,324,478,000	$ 3,166,472,000
Proceeds from debt draw	$ 200,000,000
Equity to capitalization ratio (ECR) requirement	18.00%
Other Than Long Term Debt
Credit facility information
Debt service ratio (DSR) requirement	1.02
Other
Credit facility information
Debt service ratio (DSR) requirement	1.10
2018 Revolving Credit Agreement
Credit facility information
Maximum borrowing capacity	$ 650,000,000
Available borrowing capacity	285,000,000
Outstanding borrowings	200,000,000
2018 Revolving Credit Agreement | Commercial Paper
Credit facility information
Available borrowing capacity	285,000,000
Swingline
Credit facility information
Maximum borrowing capacity	100,000,000
Letter of Credit
Credit facility information
Maximum borrowing capacity	75,000,000
Commercial Paper
Credit facility information
Maximum borrowing capacity	500,000,000
Outstanding borrowings	$ 165,000,000