Money Pool (Details) - Money Pool - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2020	Dec. 31, 2019
Short-term Debt [Line Items]
Borrowing limit	$ 250,000,000	$ 250,000,000
Amount outstanding at period end	196,000,000	39,000,000
Average amount outstanding	34,000,000	7,000,000
Maximum amount outstanding	$ 208,000,000	$ 50,000,000
Weighted average interest rate, computed on a daily basis	1.38%	2.29%
Weighted average interest rate at period end	1.15%	1.63%