Financial Instruments - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2016	Dec. 31, 2015
Liabilities:
Contingent consideration - other long-term liabilities	$ 77.6	$ 78.0
Fuel hedges
Liabilities:
Fuel hedges - other accrued liabilities	19.6	37.8
Fair Value, Measurements, Recurring
Assets:
Total assets	126.0	115.8
Liabilities:
Contingent consideration - other long-term liabilities	69.3	69.6
Total liabilities	108.4	107.4
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Assets:
Total assets	40.1	43.0
Liabilities:
Contingent consideration - other long-term liabilities	0.0	0.0
Total liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	85.9	72.8
Liabilities:
Contingent consideration - other long-term liabilities	0.0	0.0
Total liabilities	39.1	37.8
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0.0	0.0
Liabilities:
Contingent consideration - other long-term liabilities	69.3	69.6
Total liabilities	69.3	69.6
Fair Value, Measurements, Recurring | Interest rate swaps
Assets:
Interest rate swaps - other assets	31.7	16.5
Fair Value, Measurements, Recurring | Interest rate swaps | Quoted Prices in Active Markets (Level 1)
Assets:
Interest rate swaps - other assets	0.0	0.0
Fair Value, Measurements, Recurring | Interest rate swaps | Significant Other Observable Inputs (Level 2)
Assets:
Interest rate swaps - other assets	31.7	16.5
Fair Value, Measurements, Recurring | Interest rate swaps | Significant Unobservable Inputs (Level 3)
Assets:
Interest rate swaps - other assets	0.0	0.0
Fair Value, Measurements, Recurring | Fuel hedges
Liabilities:
Fuel hedges - other accrued liabilities	19.6	37.8
Fair Value, Measurements, Recurring | Fuel hedges | Quoted Prices in Active Markets (Level 1)
Liabilities:
Fuel hedges - other accrued liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Fuel hedges | Significant Other Observable Inputs (Level 2)
Liabilities:
Fuel hedges - other accrued liabilities	19.6	37.8
Fair Value, Measurements, Recurring | Fuel hedges | Significant Unobservable Inputs (Level 3)
Liabilities:
Fuel hedges - other accrued liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Interest rate locks
Liabilities:
Interest rate locks - other long-term liabilities	19.5
Fair Value, Measurements, Recurring | Interest rate locks | Quoted Prices in Active Markets (Level 1)
Liabilities:
Interest rate locks - other long-term liabilities	0.0
Fair Value, Measurements, Recurring | Interest rate locks | Significant Other Observable Inputs (Level 2)
Liabilities:
Interest rate locks - other long-term liabilities	19.5
Fair Value, Measurements, Recurring | Interest rate locks | Significant Unobservable Inputs (Level 3)
Liabilities:
Interest rate locks - other long-term liabilities	0.0
Carrying Amount
Assets:
Total assets	126.0	115.8
Liabilities:
Contingent consideration - other long-term liabilities	69.3	69.6
Total liabilities	108.4	107.4
Carrying Amount | Interest rate swaps
Assets:
Interest rate swaps - other assets	31.7	16.5
Carrying Amount | Fuel hedges
Liabilities:
Fuel hedges - other accrued liabilities	19.6	37.8
Carrying Amount | Interest rate locks
Liabilities:
Interest rate locks - other long-term liabilities	19.5
Money market mutual funds | Fair Value, Measurements, Recurring
Assets:
Money market mutual funds	40.1	43.0
Money market mutual funds | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Assets:
Money market mutual funds	40.1	43.0
Money market mutual funds | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Money market mutual funds	0.0	0.0
Money market mutual funds | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Money market mutual funds	0.0	0.0
Money market mutual funds | Carrying Amount
Assets:
Money market mutual funds	40.1	43.0
Bonds | Fair Value, Measurements, Recurring
Assets:
Bonds - restricted cash and marketable securities and other assets	54.2	56.3
Bonds | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Assets:
Bonds - restricted cash and marketable securities and other assets	0.0	0.0
Bonds | Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Bonds - restricted cash and marketable securities and other assets	54.2	56.3
Bonds | Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Bonds - restricted cash and marketable securities and other assets	0.0	0.0
Bonds | Carrying Amount
Assets:
Bonds - restricted cash and marketable securities and other assets	$ 54.2	$ 56.3