Available Credit Facilities (Details)	2 Months Ended	12 Months Ended
	Dec. 31, 2016 USD ($)	Oct. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 850,000,000	$ 850,000,000
Line of Credit Facility, Maximum Borrowing Capacity	850,000,000
Commercial paper	(330,000,000)	(145,000,000)
Outstanding letters of credit	(1,700,000)	(1,700,000)
Available capacity	518,300,000	$ 703,300,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 850,000,000
Debt Instrument, Term	5 years
Line of Credit Facility, Expiration Date	Dec. 14, 2020
Line Of Credit Facility, Optional Additional Expansion Amount	$ 200,000,000
Minimum Amount Outstanding During Period	0
Maximum Amount Outstanding During Period	$ 0
Line of Credit Facility, Covenant Terms	total debt to total capitalization of no greater than 70%
Line of Credit Facility, Covenant Compliance	actual ratio was 56%
Debt Covenant Total Debt To Total Capital Ratio	70.00%
Ratio of Indebtedness to Net Capital	0.56
Letter of Credit
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 10,000,000
Commercial Paper
Line of Credit Facility [Line Items]
Line of Credit Facility, Current Borrowing Capacity	$ 850,000,000
Short-term Debt, Weighted Average Interest Rate	0.96%	0.64%
Minimum Amount Outstanding During Period	$ 140,000,000
Maximum Amount Outstanding During Period	$ 330,000,000
Commercial Paper | Minimum
Line of Credit Facility [Line Items]
Debt Instrument, Term	7 days	1 day
Line of Credit Facility, Interest Rate During Period	0.63%
Commercial Paper | Maximum
Line of Credit Facility [Line Items]
Debt Instrument, Term	15 days	6 days
Line of Credit Facility, Interest Rate During Period	1.00%
Commercial Paper | Weighted Average
Line of Credit Facility [Line Items]
Line of Credit Facility, Interest Rate During Period	0.80%