Debt - Narrative (Details) - USD ($)	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jul. 28, 2016	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015	May 31, 2016	Jun. 30, 2014
Debt Instrument [Line Items]
Balance availability under Letters of Credit Facilities		$ 1,745,700,000		$ 1,745,700,000		$ 1,727,700,000
Letters of credit utilizing availability under Credit Facilities		485,400,000		485,400,000		503,300,000
Long-term debt		7,614,000,000		7,614,000,000		7,532,900,000
Combined Aggregate Principal of Senior Notes and Debentures Purchased		575,400,000.0
Debt instrument, face amount		7,699,100,000.0		7,699,100,000.0		7,635,800,000.0
Aggregate Amount of Premiums on Senior Notes				148,100,000
Early Tender Consideration For Purchase of Senior Notes				$ 28,700,000
Percentage of tax exempt financings				90.00%
Capital lease liabilities						111,500,000
Interest rate swaps at fair value		31,700,000		$ 31,700,000		$ 16,500,000
Net interest income related to net swap settlements		1,600,000	$ 1,900,000	3,300,000	$ 3,800,000
Loss (gain) on the change in fair value of the hedged senior notes		3,500,000	(8,200,000)	14,100,000	(2,900,000)
Gain (loss) on the related interest rate swaps		4,200,000	(8,100,000)	$ 15,200,000	(2,300,000)
Minimum
Debt Instrument [Line Items]
Range of capital lease liabilities matured				2016		2016
Maximum
Debt Instrument [Line Items]
Range of capital lease liabilities matured				2046		2046
Interest Rate Swap
Debt Instrument [Line Items]
Notional value of swap agreements		300,000,000		$ 300,000,000
Expected amortization of yield adjustment on senior notes				1,600,000
Interest rate locks
Debt Instrument [Line Items]
Notional value of swap agreements		$ 525,000,000		$ 525,000,000
Derivative, interest rate floor		1.90%		1.90%
Derivative, interest rate cap		2.2795%		2.2795%
Accumulated and other comprehensive income, net of tax		$ 11,800,000		$ 11,800,000
Effective portion of interest rate locks amortized as an adjustment to interest expense		1,000,000	$ 600,000	1,300,000	$ 1,300,000
Interest rate locks | Long-term Debt
Debt Instrument [Line Items]
Interest rate locks fair value		19,500,000		19,500,000
Line Of Credit Due May 2021
Debt Instrument [Line Items]
Revolving credit facility maximum borrowing capacity							$ 1,000,000,000
Aggregate additional borrowings through increased commitments from existing lenders				500,000,000
Line Of Credit Due May 2017
Debt Instrument [Line Items]
Revolving credit facility maximum borrowing capacity		1,250,000,000.00		1,250,000,000.00
Reduction in maximum borrowing capacity								$ 1,000,000,000
Line Of Credit Due June 2019
Debt Instrument [Line Items]
Line of credit facility, increase				500,000,000
Long-term debt		0		0		$ 0
Debt instrument, face amount		0		0		0
Uncommitted Credit Facility
Debt Instrument [Line Items]
Revolving credit facility maximum borrowing capacity		125,000,000		125,000,000
Uncommitted Revolver
Debt Instrument [Line Items]
Long-term debt		84,500,000		84,500,000		19,000,000
Debt instrument, face amount		84,500,000.0		84,500,000.0		19,000,000.0
Senior Notes 6.20% Due March 2040
Debt Instrument [Line Items]
Long-term debt		$ 643,500,000		$ 643,500,000		643,400,000
Interest rate (in percentage)		6.20%		6.20%
Debt instrument, face amount		$ 650,000,000		$ 650,000,000		650,000,000
Senior Notes Due May 2041
Debt Instrument [Line Items]
Long-term debt		$ 591,200,000		$ 591,200,000		591,100,000
Interest rate (in percentage)		5.70%		5.70%
Debt instrument, face amount		$ 600,000,000		$ 600,000,000		600,000,000
Senior Notes Due March 2035
Debt Instrument [Line Items]
Long-term debt		$ 252,100,000		$ 252,100,000		251,800,000
Interest rate (in percentage)		6.086%		6.086%
Debt instrument, face amount		$ 275,700,000.0		$ 275,700,000.0		275,700,000.0
Debentures $360.0 million 7.400% Due September 2035
Debt Instrument [Line Items]
Long-term debt		$ 125,600,000		$ 125,600,000		125,300,000
Interest rate (in percentage)		7.40%		7.40%
Debt instrument, face amount		$ 165,200,000.0		$ 165,200,000.0		165,200,000.0
Senior Notes $500.0 million 2.90% due 2026
Debt Instrument [Line Items]
Interest rate (in percentage)		2.90%		2.90%
Debt instrument, face amount		$ 500,000,000		$ 500,000,000
Senior Notes $500.0 million 3.20% Due March 2025
Debt Instrument [Line Items]
Long-term debt		$ 494,200,000		$ 494,200,000		494,000,000
Interest rate (in percentage)		3.20%		3.20%
Debt instrument, face amount		$ 500,000,000		$ 500,000,000		$ 500,000,000
Senior Notes $500.0 million 3.20% Due March 2025 | Interest rate locks
Debt Instrument [Line Items]
Interest rate (in percentage)						3.20%
Accumulated and other comprehensive income, net of tax		18,700,000		18,700,000		$ 19,400,000
Tax Exempt Bonds And Other Tax Exempt Financings
Debt Instrument [Line Items]
Long-term debt		1,072,400,000		1,072,400,000		1,072,100,000
Debt instrument, face amount		$ 1,079,100,000.0		$ 1,079,100,000.0		1,079,100,000.0
Tax Exempt Bonds And Other Tax Exempt Financings | Minimum
Debt Instrument [Line Items]
Interest rate (in percentage)		0.65%		0.65%
Tax Exempt Bonds And Other Tax Exempt Financings | Maximum
Debt Instrument [Line Items]
Interest rate (in percentage)		5.625%		5.625%
Senior Notes $550.0 million 4.750% Due May 2023
Debt Instrument [Line Items]
Long-term debt		$ 573,800,000		$ 573,800,000		559,400,000
Interest rate (in percentage)		4.75%		4.75%
Debt instrument, face amount		$ 550,000,000		$ 550,000,000		$ 550,000,000
Subsequent Event
Debt Instrument [Line Items]
Gain (Loss) on Extinguishment of Debt	$ 200,000,000