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

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Description of the interest rate as being fixed or variable, and, if variable, identification of the index or rate on which the interest rate is based and the number of points or percentage added to that index or rate to set the rate, and other pertinent information, such as frequency of rate resets.

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

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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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Amount, after accumulated amortization, of debt discount (premium).

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

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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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Debt Schedule (Detail) - USD ($) $ in Millions	12 Months Ended
Debt Schedule (Detail) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Debt Instrument
Long-term Debt, Excluding Current Maturities		$ 2,738.4		$ 2,999.4
Debt Instrument, Unamortized Premium (Discount), Net		9.7		9.8
Debt Issuance Costs, Noncurrent, Net		(24.4)		(28.4)
Current Maturities of Long-term Debt		199.9		0.0
Debt Issuance Costs, Current, Net		(0.1)		0.0
Long-term Debt		2,938.3		2,999.4
Fair Value Hedge Adjustment, LT Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities		(4.5)		(3.1)
Secured Notes Issued in 2007 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		200.0		260.0
Interest Rate Terms	Variable
Year of Maturity	Dec. 31, 2037
Secured Notes acquired 2016 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		0.0		3.5
Interest Rate Terms	Variable
Year of Maturity	Apr. 08, 2038
Notes Issued in 1998 with 2018 Maturity Date \| Senior Notes
Debt Instrument
Current Maturities of Long-term Debt		200.0		0.0
Stated Interest Rate of Debt			7.00%
Year of Maturity	Dec. 31, 2018
Notes Issued in 1998 with 2018 Maturity Date \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		0.0		200.0
Stated Interest Rate of Debt			7.00%
Year of Maturity	Dec. 31, 2018
Notes Issued in 1998 with 2028 Maturity Date \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		365.8		365.8
Interest Rate Terms	6.750 - 7.250%
Year of Maturity	Dec. 31, 2028
Notes Issued in 1998 with 2028 Maturity Date \| Senior Notes \| Minimum
Debt Instrument
Stated Interest Rate of Debt			6.75%
Notes Issued in 1998 with 2028 Maturity Date \| Senior Notes \| Maximum
Debt Instrument
Stated Interest Rate of Debt			7.25%
Notes Issued in 2002 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		39.5		39.5
Stated Interest Rate of Debt			7.375%
Year of Maturity	Dec. 31, 2032
Notes Issued in 2010 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		400.0		400.0
Stated Interest Rate of Debt			5.625%
Year of Maturity	Dec. 31, 2020
Notes Payable due 2042
Debt Instrument
Stated Interest Rate of Debt			5.75%
Notes Payable due 2042 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		500.0		500.0
Stated Interest Rate of Debt			5.75%
Year of Maturity	Dec. 31, 2042
Notes Issued in 2014 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		350.0		350.0
Stated Interest Rate of Debt			4.00%
Year of Maturity	Dec. 31, 2024
Notes Issued in 2015 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		275.0		275.0
Stated Interest Rate of Debt			3.875%
Year of Maturity	Dec. 31, 2025
Notes Issued in 2016 \| Senior Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		350.0		350.0
Stated Interest Rate of Debt			3.00%
Year of Maturity	May 15, 2021
Notes Issued 1990 to 1996 \| Medium-term Notes
Debt Instrument
Long-term Debt, Excluding Current Maturities		50.8		50.8
Interest Rate Terms	7.000 - 7.190%
Debt Instrument, Maturity Date Range, Start	Dec. 31, 2023
Debt Instrument, Maturity Date Range, End	Dec. 31, 2028
Notes Issued 1990 to 1996 \| Medium-term Notes \| Minimum
Debt Instrument
Stated Interest Rate of Debt			7.00%
Notes Issued 1990 to 1996 \| Medium-term Notes \| Maximum
Debt Instrument
Stated Interest Rate of Debt			7.19%
Notes Issued in 1998 with 2038 Maturity Date \| Junior Subordinated Debt
Debt Instrument
Long-term Debt, Excluding Current Maturities		$ 226.5		$ 226.5
Stated Interest Rate of Debt	7.405%		7.405%
Year of Maturity	Dec. 31, 2038