Debt and Credit Agreements Long-Term Debt (Details) - USD ($)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019
Debt Instrument [Line Items]
Long-term debt, gross	$ 3,418,300,000	$ 3,273,900,000
Long-term debt, current maturities	502,500,000	502,000,000.0
Long-term debt, excluding current maturities	2,915,800,000	2,771,900,000
Deb instrument, unamortized debt issuance costs	24,300,000	21,600,000
Long-term debt, fair value	3,995,000,000.0	3,565,500,000
3.50% Notes Due 2020
Debt Instrument [Line Items]
Senior Notes	500,000,000.0
Long-term debt, gross	$ 0	498,500,000
Interest rate, stated percentage	3.50%
Debt instrument, interest rate, effective percentage	3.89%
3.75% Notes Due 2021 (less unamortized discount and issuance costs of $0.1 and $0.8, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 500,000,000.0
Long-term debt, gross	499,100,000	497,900,000
Debt instrument, unamortized discount	100,000
Deb instrument, unamortized debt issuance costs	$ 800,000
Debt instrument, maturity date	Oct. 01, 2021
Interest rate, stated percentage	3.75%
Debt instrument, interest rate, effective percentage	3.98%
4.00% Notes Due 2022 (less unamortized discount and issuance costs of $0.4 and $0.3, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 250,000,000.0
Long-term debt, gross	249,300,000	248,700,000
Debt instrument, unamortized discount	400,000
Deb instrument, unamortized debt issuance costs	$ 300,000
Debt instrument, maturity date	Mar. 15, 2022
Interest rate, stated percentage	4.00%
Debt instrument, interest rate, effective percentage	4.13%
3.75% Notes Due 2023 (less unamortized discount and issuance costs of $0.3 and $0.9, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 500,000,000.0
Long-term debt, gross	498,800,000	498,200,000
Debt instrument, unamortized discount	300,000
Deb instrument, unamortized debt issuance costs	$ 900,000
Debt instrument, maturity date	Feb. 15, 2023
Interest rate, stated percentage	3.75%
Debt instrument, interest rate, effective percentage	4.32%
4.20% Notes Due 2024 (less unamortized discount and issuance costs of $0.3 and $1.4, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 500,000,000.0
Long-term debt, gross	498,300,000	497,800,000
Debt instrument, unamortized discount	300,000
Deb instrument, unamortized debt issuance costs	$ 1,400,000
Debt instrument, maturity date	Apr. 15, 2024
Interest rate, stated percentage	4.20%
Debt instrument, interest rate, effective percentage	4.24%
4.65% Notes Due 2028 (less unamortized discount and issuance costs of $1.3 and $3.5, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 500,000,000.0
Long-term debt, gross	495,200,000	494,600,000
Debt instrument, unamortized discount	1,300,000
Deb instrument, unamortized debt issuance costs	$ 3,500,000
Debt instrument, maturity date	Oct. 01, 2028
Interest rate, stated percentage	4.65%
Debt instrument, interest rate, effective percentage	4.78%
4.75% Note Due 2030 (less unamortized discount and issuance costs of $3.5 and $5.7, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 650,000,000.0
Long-term debt, gross	640,800,000	0
Debt instrument, unamortized discount	3,500,000
Deb instrument, unamortized debt issuance costs	$ 5,700,000
Debt instrument, maturity date	Mar. 30, 2030
Interest rate, stated percentage	4.75%
Debt instrument, interest rate, effective percentage	4.92%
5.40% Notes Due 2048 (less unamortized discount and issuance costs of $2.7 and $5.2, respectively)
Debt Instrument [Line Items]
Senior Notes	$ 500,000,000.0
Long-term debt, gross	492,100,000	491,900,000
Debt instrument, unamortized discount	2,700,000
Deb instrument, unamortized debt issuance costs	$ 5,200,000
Debt instrument, maturity date	Oct. 01, 2048
Interest rate, stated percentage	5.40%
Debt instrument, interest rate, effective percentage	5.48%
Other notes payable and capitalized leases
Debt Instrument [Line Items]
Long-term debt, gross	$ 44,700,000	$ 46,300,000

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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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.

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- Definition

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, 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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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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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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