Debt (Details)		12 Months Ended
	Dec. 01, 2016 USD ($)	Dec. 31, 2016 USD ($) line_of_credit financial_institution	Dec. 31, 2015 USD ($)
Short-term Debt [Line Items]
Short-term debt outstanding		$ 512,970,000	$ 581,700,000
Commercial Paper [Member]
Short-term Debt [Line Items]
Short-term debt outstanding		$ 513,000,000	$ 581,700,000
Weighted average interest rates on short-term debt outstanding		0.90%	0.50%
Average daily borrowings		$ 422,900,000	$ 387,900,000
Weighted average interest rates on average daily borrowings		0.60%	0.30%
Credit facility, maximum borrowing capacity		$ 1,000,000,000.0
Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility, maximum borrowing capacity		1,000,000,000.0
Incremental increases in principal		250,000,000.0
Amounts outstanding under credit facility		0	$ 0
Proceeds from lines of credit		0	0
Credit Facility [Member] | SCB LLC [Member]
Short-term Debt [Line Items]
Incremental increases in principal		$ 225,000,000
Number of uncommitted lines of credit | line_of_credit		4
Number of financial institutions offering lines of credit | financial_institution		4
Number of financial institutions with stated limits | line_of_credit		3
Number of financial institutions with no stated limits | line_of_credit		1
Uncommitted Lines of Credit [Member]
Short-term Debt [Line Items]
Short-term debt outstanding		$ 0	0
Average daily borrowings		$ 4,400,000	$ 3,900,000
Weighted average interest rates on average daily borrowings		1.10%	1.20%
Credit Facility [Member] | Revolving Credit Facility [Member]
Short-term Debt [Line Items]
Credit facility, maximum borrowing capacity	$ 200,000,000
Amounts outstanding under credit facility		$ 0
Term of debt instrument	364 days
Average daily borrowings		$ 7,300,000
Weighted average interest rate		1.60%