Fair Value Measurements - Financial Instruments (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Fair value of financial instruments
Preferred stock of subsidiary	$ 30.4	$ 30.4
Long-term debt, including current portion	11,858.3	10,335.7
Finance lease obligation	45.9	23.3
Carrying amount
Fair value of financial instruments
Preferred stock of subsidiary	30.4	30.4
Long-term debt, including current portion	11,858.3	10,335.7
Finance lease obligation	45.9
Capital lease obligation		23.3
Fair value
Fair value of financial instruments
Preferred stock of subsidiary	29.5	28.3
Long-term debt, including current portion	$ 13,035.9	$ 10,554.9

Amount equal to the present value (the principal) at the beginning of the lease term of minimum lease payments during the lease term (excluding that portion of the payments representing executory costs such as insurance, maintenance, and taxes to be paid by the lessor, together with any profit thereon) net of payments or other amounts applied to the principal through the balance sheet date.

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Present value of lessee's discounted obligation for lease payments from finance lease.

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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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Aggregate par or stated value of issued nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer). This item includes treasury stock repurchased by the entity. Note: elements for number of nonredeemable preferred shares, par value and other disclosure concepts are in another section within stockholders' equity.

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