Money Pool (Details) - USD ($)	3 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Short-term Debt [Line Items]
Amount outstanding at period end	$ 0	$ 307,000,000.0
Money Pool
Short-term Debt [Line Items]
Borrowing limit	250,000,000	250,000,000
Amount outstanding at period end	0	0
Average amount outstanding	23,000,000	25,000,000
Maximum amount outstanding	$ 50,000,000	$ 156,000,000
Weighted average interest rate, computed on a daily basis	2.29%	1.93%