Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Assets:
Marketable debt securities	$ 5,172.5	$ 5,398.0
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,229.4	2,039.6
Marketable equity securities	11.8	24.9
Derivative contracts	2.7	61.0
Plan assets for deferred compensation	28.5	34.5
Total	6,444.9	7,558.0
Liabilities:
Derivative contracts	111.3	13.6
Contingent consideration obligations	523.6	467.6	$ 506.0
Total	634.9	481.2
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,609.8	2,663.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,919.3	2,172.5
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	643.4	561.7
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	11.8	24.9
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	11.8	24.9
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	1,229.4	2,039.6
Marketable equity securities	0.0	0.0
Derivative contracts	2.7	61.0
Plan assets for deferred compensation	28.5	34.5
Total	6,433.1	7,533.1
Liabilities:
Derivative contracts	111.3	13.6
Contingent consideration obligations	0.0	0.0
Total	111.3	13.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,609.8	2,663.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,919.3	2,172.5
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	643.4	561.7
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	523.6	467.6
Total	523.6	467.6
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