Debt - Summary of Debt (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Debt Instrument [Line Items]
Long-term debt	$ 2,495.8	$ 2,645.7
Less current portion	0.0	150.0
Less unamortized debt issuance costs	14.6	15.3
Long-term debt	$ 2,481.2	$ 2,480.4
Weighted-average interest rate	3.64%	3.80%
6.50% Senior Notes due March 2018 \| Senior Notes
Debt Instrument [Line Items]
Long-term debt		$ 150.0
Stated interest rate		6.50%
2.45% Senior Notes, due December 2020 \| Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 499.5	$ 499.5
Stated interest rate	2.45%	2.45%
3.90% Senior Notes, due June 2022 \| Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 399.8	$ 399.8
Stated interest rate	3.90%	3.90%
4.50% Senior Notes, due November 2023 \| Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 698.9	$ 698.8
Stated interest rate	4.50%	4.50%
3.65% Senior Notes, due September 2024 \| Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 399.2	$ 399.2
Stated interest rate	3.65%	3.65%
3.40% Senior Notes, due December 2027 \| Senior Notes
Debt Instrument [Line Items]
Long-term debt	$ 498.4	$ 498.4
Stated interest rate	3.40%	3.40%
Current Portion of Long-term Debt
Debt Instrument [Line Items]
Weighted-average interest rate	0.00%	6.50%
Long-term Debt, Excluding Current Portion
Debt Instrument [Line Items]
Weighted-average interest rate	3.64%	3.64%

Contractual interest rate for funds borrowed, under the debt agreement.

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 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Weighted average interest rate of debt outstanding.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28555-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
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-Section S99
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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: 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 2: 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(16))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

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Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

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-Name Accounting Standards Codification
-Topic 210
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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.

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Reference 1: http://www.xbrl.org/2003/role/presentationRef
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