Debt - Debt and Average Interest Rates on Debt Outstanding (Detail) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized debt issuance costs and discounts	$ (5.5)	$ (6.8)
Total debt	798.8	1,440.7
Long-term debt	$ 798.8	1,440.7
Commercial Paper
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	2.934%
Commercial paper	$ 99.0	34.0
Line of Credit \| Revolving Credit Facility
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	3.461%
Total debt	$ 9.4	28.4
Senior Notes \| Senior Notes 2.900% Due 2018
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	2.90%
Total debt	$ 0.0	255.3
Senior Notes \| Senior Notes 2.650% Due 2019
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	2.65%
Total debt	$ 250.0	250.0
Senior Notes \| Senior Notes 2.45% Due 2019
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	2.45%
Total debt	$ 160.4	594.4
Senior Notes \| Senior Notes 3.625% Due 2020
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	3.625%
Total debt	$ 74.0	74.0
Senior Notes \| Senior Notes 5.000% Due 2021
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	5.00%
Total debt	$ 103.8	103.8
Senior Notes \| Senior Notes 3.150% Due 2022
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	3.15%
Total debt	$ 88.3	88.3
Senior Notes \| Senior Notes 4.650% Due 2025
Debt Instrument [Line Items]
Average interest rate as of September 30, 2018	4.65%
Total debt	$ 19.3	19.3
Other
Debt Instrument [Line Items]
Total debt	$ 0.1	$ 0.0

Carrying value as of the balance sheet date of short-term borrowings using unsecured obligations issued by banks, corporations and other borrowers to investors. The maturities of these money market securities generally do not exceed 270 days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(13)(2))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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Represents the aggregate of total long-term debt, including current maturities and short-term debt.

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- Definition

Weighted average interest rate of debt outstanding.

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- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
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