Financial Instruments - Assets and Liabilities Measured at Fair Value (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Liabilities:
Contingent consideration	$ 68.2	$ 73.9
Fair Value, Measurements, Recurring
Assets:
Total assets	101.0	97.7
Liabilities:
Interest rate swaps	32.4
Contingent consideration	72.1	71.4
Total liabilities	104.5	71.4
Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Assets:
Total assets	38.6	37.1
Liabilities:
Interest rate swaps	0.0
Contingent consideration	0.0	0.0
Total liabilities	0.0	0.0
Fair Value, Measurements, Recurring | Significant Other Observable Inputs (Level 2)
Assets:
Total assets	62.4	60.6
Liabilities:
Interest rate swaps	32.4
Contingent consideration	0.0	0.0
Total liabilities	32.4	0.0
Fair Value, Measurements, Recurring | Significant Unobservable Inputs (Level 3)
Assets:
Total assets	0.0	0.0
Liabilities:
Interest rate swaps	0.0
Contingent consideration	72.1	71.4
Total liabilities	72.1	71.4
Fair Value, Measurements, Recurring | Interest Rate Swap
Assets:
Interest rate derivatives	11.6	2.5
Fair Value, Measurements, Recurring | Interest Rate Swap | Quoted Prices in Active Markets (Level 1)
Assets:
Interest rate derivatives	0.0	0.0
Fair Value, Measurements, Recurring | Interest Rate Swap | Significant Other Observable Inputs (Level 2)
Assets:
Interest rate derivatives	11.6	2.5
Fair Value, Measurements, Recurring | Interest Rate Swap | Significant Unobservable Inputs (Level 3)
Assets:
Interest rate derivatives	0.0	0.0
Fair Value, Measurements, Recurring | Interest Rate Swap Locks
Assets:
Interest rate derivatives		10.3
Derivatives interest rate locks	0.3
Fair Value, Measurements, Recurring | Interest Rate Swap Locks | Quoted Prices in Active Markets (Level 1)
Assets:
Interest rate derivatives		0.0
Derivatives interest rate locks	0.0
Fair Value, Measurements, Recurring | Interest Rate Swap Locks | Significant Other Observable Inputs (Level 2)
Assets:
Interest rate derivatives	0.3	10.3
Derivatives interest rate locks	0.3
Fair Value, Measurements, Recurring | Interest Rate Swap Locks | Significant Unobservable Inputs (Level 3)
Assets:
Interest rate derivatives		0.0
Derivatives interest rate locks	0.0
Carrying Amount
Assets:
Total assets	101.0	97.7
Liabilities:
Interest rate swaps	32.4
Contingent consideration	72.1	71.4
Total liabilities	104.5	71.4
Carrying Amount | Interest Rate Swap
Assets:
Interest rate derivatives	11.6	2.5
Carrying Amount | Interest Rate Swap Locks
Assets:
Interest rate derivatives		10.3
Derivatives interest rate locks	0.3
Money market mutual funds | Fair Value, Measurements, Recurring
Assets:
Money market mutual funds	38.6	37.1
Money market mutual funds | Fair Value, Measurements, Recurring | Quoted Prices in Active Markets (Level 1)
Assets:
Money market mutual funds	38.6	37.1
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	38.6	37.1
Bonds | Fair Value, Measurements, Recurring
Assets:
Bonds - restricted cash and marketable securities and other assets	50.5	47.8
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	50.5	47.8
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	$ 50.5	$ 47.8