Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Assets:
Marketable debt securities	$ 3,689.3	$ 5,172.5
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	705.5	1,229.4
Marketable equity securities	615.4	11.8
Derivative contracts	66.9	2.7
Plan assets for deferred compensation	25.4	28.5
Total	5,102.5	6,444.9
Liabilities:
Derivative contracts	24.6	111.3
Contingent consideration obligations	409.8	523.6	$ 467.6
Total	434.4	634.9
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,459.2	2,609.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	969.6	1,919.3
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	260.5	643.4
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	51.7	11.8
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	51.7	11.8
Liabilities:
Derivative contracts	0.0	0.0
Contingent consideration obligations	0.0	0.0
Total	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	0.0	0.0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	0.0	0.0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	705.5	1,229.4
Marketable equity securities	563.7	0.0
Derivative contracts	66.9	2.7
Plan assets for deferred compensation	25.4	28.5
Total	5,050.8	6,433.1
Liabilities:
Derivative contracts	24.6	111.3
Contingent consideration obligations	0.0	0.0
Total	24.6	111.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,459.2	2,609.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	969.6	1,919.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	260.5	643.4
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	0.0	0.0
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	0.0	0.0
Liabilities:
Derivative contracts	0.0	0.0
Contingent consideration obligations	409.8	523.6
Total	409.8	523.6
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	0.0	0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	$ 0.0	$ 0.0