Debt - Carrying Values of Debt Instruments (Details) - USD ($)	9 Months Ended
Sep. 30, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Principal	$ 7,820,800,000.0	$ 7,635,800,000.0
Adjustments	(75,500,000)	(102,900,000)
Capital leases		111,500,000
Carrying Value	7,745,300,000	7,532,900,000
Less: current portion	(5,700,000)	(5,500,000)
Long-term portion	$ 7,739,600,000	7,527,400,000
Uncommitted Revolver
Debt Instrument [Line Items]
Interest Rate	Variable
Principal	$ 73,000,000	19,000,000.0
Adjustments	0	0
Carrying Value	$ 73,000,000	19,000,000
Credit facilities: June 2019
Debt Instrument [Line Items]
Interest Rate	Variable
Principal	$ 70,000,000	0
Adjustments	0	0
Carrying Value	$ 70,000,000	0
Credit facilities: May 2021
Debt Instrument [Line Items]
Interest Rate	Variable
Principal	$ 140,000,000	0
Adjustments	0	0
Carrying Value	$ 140,000,000	0
Senior notes: May 2018
Debt Instrument [Line Items]
Interest rate (in percentage)	3.80%
Principal	$ 700,000,000	700,000,000
Adjustments	(1,400,000)	(2,000,000)
Carrying Value	$ 698,600,000	698,000,000
Senior notes: September 2019
Debt Instrument [Line Items]
Interest rate (in percentage)	5.50%
Principal	$ 650,000,000	650,000,000
Adjustments	(3,500,000)	(4,400,000)
Carrying Value	$ 646,500,000	645,600,000
Senior notes: March 2020
Debt Instrument [Line Items]
Interest rate (in percentage)	5.00%
Principal	$ 850,000,000	850,000,000
Adjustments	(2,800,000)	(3,400,000)
Carrying Value	$ 847,200,000	846,600,000
Senior notes: November 2021
Debt Instrument [Line Items]
Interest rate (in percentage)	5.25%
Principal	$ 600,000,000	600,000,000
Adjustments	(2,000,000)	(2,300,000)
Carrying Value	$ 598,000,000	597,700,000
Senior notes: June 2022
Debt Instrument [Line Items]
Interest rate (in percentage)	3.55%
Principal	$ 850,000,000	850,000,000
Adjustments	(5,800,000)	(6,500,000)
Carrying Value	$ 844,200,000	843,500,000
Senior notes: May 2023
Debt Instrument [Line Items]
Interest rate (in percentage)	4.75%
Principal	$ 550,000,000	550,000,000
Adjustments	19,400,000	9,400,000
Carrying Value	$ 569,400,000	559,400,000
Senior notes: March 2025
Debt Instrument [Line Items]
Interest rate (in percentage)	3.20%
Principal	$ 500,000,000	500,000,000
Adjustments	(5,600,000)	(6,000,000)
Carrying Value	$ 494,400,000	494,000,000
Senior notes: June 2026
Debt Instrument [Line Items]
Interest rate (in percentage)	2.90%
Principal	$ 500,000,000	0
Adjustments	(5,600,000)	0
Carrying Value	$ 494,400,000	0
Senior notes: March 2035
Debt Instrument [Line Items]
Interest rate (in percentage)	6.086%
Principal	$ 181,900,000.0	275,700,000.0
Adjustments	(15,600,000)	(23,900,000)
Carrying Value	$ 166,300,000	251,800,000
Senior notes: March 2040
Debt Instrument [Line Items]
Interest rate (in percentage)	6.20%
Principal	$ 399,900,000.0	650,000,000
Adjustments	(4,000,000)	(6,600,000)
Carrying Value	$ 395,900,000	643,400,000
Senior notes: May 2041
Debt Instrument [Line Items]
Interest rate (in percentage)	5.70%
Principal	$ 385,700,000.0	600,000,000
Adjustments	(5,600,000)	(8,900,000)
Carrying Value	$ 380,100,000	591,100,000
Debentures: May 2021
Debt Instrument [Line Items]
Interest rate (in percentage)	9.25%
Principal	$ 35,300,000.0	35,300,000.0
Adjustments	(1,200,000)	(1,400,000)
Carrying Value	$ 34,100,000	33,900,000
Debentures: September 2035
Debt Instrument [Line Items]
Interest rate (in percentage)	7.40%
Principal	$ 148,100,000.0	165,200,000.0
Adjustments	(35,300,000)	(39,900,000)
Carrying Value	112,800,000	125,300,000
Tax-exempt: 2019 - 2044
Debt Instrument [Line Items]
Principal	1,079,100,000.0	1,079,100,000.0
Adjustments	(6,500,000)	(7,000,000)
Carrying Value	1,072,600,000	1,072,100,000
Capital leases: 2016 - 2046
Debt Instrument [Line Items]
Principal	107,800,000.0	111,500,000.0
Adjustments	0	0
Capital leases	$ 107,800,000	$ 111,500,000
Minimum \| Tax-exempt: 2019 - 2044
Debt Instrument [Line Items]
Interest rate (in percentage)	0.50%
Minimum \| Capital leases: 2016 - 2046
Debt Instrument [Line Items]
Interest rate (in percentage)	4.00%
Maximum \| Tax-exempt: 2019 - 2044
Debt Instrument [Line Items]
Interest rate (in percentage)	5.625%
Maximum \| Capital leases: 2016 - 2046
Debt Instrument [Line Items]
Interest rate (in percentage)	12.203%

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 and due to be paid more than one year (or one operating cycle, if longer) after the balance sheet date.

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-Publisher FASB
-Name Accounting Standards Codification
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- Definition

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

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- Definition

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

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Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions).

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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 of long-term debt and capital lease obligation due after one year or beyond the normal operating cycle, if longer.

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Amount of long-term debt and capital leases due within one year or the normal operating cycle, if longer.

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