Long-term Debt (Details) - USD ($) $ in Millions	3 Months Ended
Jun. 30, 2019	Dec. 31, 2018
Debt Instrument [Line Items]
Long-term Debt, Gross	$ 3,564.1	$ 3,660.3
Long-term Debt, Current Maturities	0.3	0.1
Long-term Debt, Excluding Current Maturities	3,563.8	3,660.2
3.50% Notes Due 2020 (less unamortized discount and issuance costs of $0.6 and $1.8, respectively)
Debt Instrument [Line Items]
Par Value	$ 500.0
Maturity Date	Oct. 01, 2020
Interest Rate Stated Percentage	3.50%
Effective Interest Rate	3.89%
Unamortized Discount	$ 0.6
Issuance Cost	1.8
Long-term Debt, Gross	497.6	496.6
Long-term Debt, Fair Value	$ 506.3	499.9
3.75% Notes Due 2021 (less unamortized discount and issuance costs of $0.3 and $2.4, respectively)
Debt Instrument [Line Items]
Maturity Date	Oct. 01, 2021
Interest Rate Stated Percentage	3.75%
Effective Interest Rate	3.98%
Unamortized Discount	$ 0.3
Issuance Cost	2.4
Long-term Debt, Gross	497.3	496.8
Long-term Debt, Fair Value	513.6	503.2
4.00% Notes Due 2022 (less unamortized discount and issuance costs of $0.8 and $0.7, respectively)
Debt Instrument [Line Items]
Par Value	$ 250.0
Maturity Date	Mar. 15, 2022
Interest Rate Stated Percentage	4.00%
Effective Interest Rate	4.13%
Unamortized Discount	$ 0.8
Issuance Cost	0.7
Long-term Debt, Gross	248.5	248.2
Long-term Debt, Fair Value	258.5	250.3
3.75% Notes Due 2023 (less unamortized discount and issuance costs of $0.6 and $1.5, respectively)
Debt Instrument [Line Items]
Par Value	$ 500.0
Maturity Date	Feb. 15, 2023
Interest Rate Stated Percentage	3.75%
Effective Interest Rate	4.32%
Unamortized Discount	$ 0.6
Issuance Cost	1.5
Long-term Debt, Gross	497.9	497.7
Long-term Debt, Fair Value	520.7	491.4
4.20% Notes Due 2024 (less unamortized discount and issuance costs of $0.5 and $2.0, respectively)
Debt Instrument [Line Items]
Par Value	$ 500.0
Maturity Date	Apr. 15, 2024
Interest Rate Stated Percentage	4.20%
Effective Interest Rate	4.24%
Unamortized Discount	$ 0.5
Issuance Cost	2.0
Long-term Debt, Gross	497.5	497.3
Long-term Debt, Fair Value	538.2	492.6
4.65% Notes Due 2028 (less unamortized discount and issuance costs of $1.6 and $4.1, respectively)
Debt Instrument [Line Items]
Par Value	$ 500.0
Maturity Date	Oct. 01, 2028
Interest Rate Stated Percentage	4.65%
Effective Interest Rate	4.78%
Unamortized Discount	$ 1.6
Issuance Cost	4.1
Long-term Debt, Gross	494.3	494.0
Long-term Debt, Fair Value	545.9	494.1
5.40% Notes Due 2048 (less unamortized discount and issuance costs of $2.7 and $5.5, respectively)
Debt Instrument [Line Items]
Par Value	$ 500.0
Maturity Date	Oct. 01, 2048
Interest Rate Stated Percentage	5.40%
Effective Interest Rate	5.48%
Unamortized Discount	$ 2.7
Issuance Cost	5.5
Long-term Debt, Gross	491.8	491.7
Long-term Debt, Fair Value	555.6	474.1
Term Loan due 2021 - LIBOR plus 1.25%
Debt Instrument [Line Items]
Long-term Debt, Gross	300.0	400.0
Long-term Debt	300.0
Other notes payable and capitalized leases
Debt Instrument [Line Items]
Long-term Debt, Gross	39.2	38.0
Long-term Debt, Fair Value	$ 39.2	$ 38.0

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

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Face (par) amount of debt instrument at time of issuance.

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Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

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

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Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount.

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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.

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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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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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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.

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