Fair Value of Financial Assets and Liabilities, Changes in Level 3 Commodity Derivatives (Details) (Details) - Commodity Contract - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Fair Value, Net Derivative Asset (Liability) Measured on Recurring Basis, Unobservable Input Reconciliation [Roll Forward]
Balance at beginning of period	$ 12,700,000	$ 2,000,000	$ 5,100,000
Purchases	32,300,000	41,200,000	7,600,000
Settlements	(41,600,000)	(55,800,000)	(41,900,000)
Gains (losses) recognized as regulatory assets	11,300,000	25,300,000	31,200,000
Balance at end of period	14,700,000	12,700,000	2,000,000
Transfers into Level 3	0	0	0
Transfers out of Level 3	$ 0	$ 0	$ 0