Debt - Debt and Average Interest Rates on Debt Outstanding (Detail) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Unamortized Debt Issuance Expense	$ (12.9)	$ (6.8)
Total debt	2,673.3	1,440.7
Current maturities and short-term borrowings	(0.2)	0.0
Long-term debt	$ 2,673.1	1,440.7
Commercial Paper
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.34%
Commercial Paper	$ 257.8	34.0
Senior Notes 2.900% Due 2018 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.90%
Total debt	$ 255.3	255.3
Senior Notes 2.650% Due 2019 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.65%
Total debt	$ 250.0	250.0
Senior Notes 2.45% Due 2019 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	2.45%
Total debt	$ 615.4	594.4
Senior Notes 3.625% Due 2020 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.625%
Total debt	$ 74.0	74.0
Senior Notes 5.000% Due 2021 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	5.00%
Total debt	$ 103.8	103.8
Senior Notes 3.150% Due 2022 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.15%
Total debt	$ 88.3	88.3
Senior Notes 4.650% Due 2025 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.65%
Total debt	$ 19.3	19.3
Senior Notes 3.950% Due 2023 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.95%
Total debt	$ 300.0	0.0
Senior Notes 4.550% Due 2028 \| Senior Notes
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	4.55%
Total debt	$ 500.0	0.0
Other Debt Obligations \| Other Debt Obligations
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	8.545%
Total debt	$ 0.2	0.0
Revolving Credit Facility \| Long Term Revolving Credit Facility
Debt Instrument [Line Items]
Debt, Weighted Average Interest Rate	3.43%
Total debt	$ 222.1	$ 28.4

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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(3))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of short-term debt and current maturity of long-term debt and capital lease obligations due within one year or the normal operating cycle, if longer.

+ References

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Represents the aggregate of total long-term debt, including current maturities and short-term debt.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtLongtermAndShorttermCombinedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Weighted average interest rate of debt outstanding.

+ References

No definition available.

+ Details

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Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- 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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

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

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant