Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets:
Marketable debt securities	$ 3,008.2	$ 5,172.5
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	3,773.7	1,229.4
Marketable equity securities	22.8	11.8
Derivative contracts	3.2	2.7
Plan assets for deferred compensation	28.3	28.5
Total	6,836.2	6,444.9
Liabilities:
Derivative contracts	147.7	111.3
Contingent consideration obligations	498.0	523.6	$ 470.9	$ 467.6
Total	645.7	634.9
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,557.9	2,609.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,143.2	1,919.3
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	307.1	643.4
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	22.8	11.8
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	22.8	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	3,773.7	1,229.4
Marketable equity securities	0.0	0.0
Derivative contracts	3.2	2.7
Plan assets for deferred compensation	28.3	28.5
Total	6,813.4	6,433.1
Liabilities:
Derivative contracts	147.7	111.3
Contingent consideration obligations	0.0	0.0
Total	147.7	111.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,557.9	2,609.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,143.2	1,919.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	307.1	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	498.0	523.6
Total	498.0	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