Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2017	Dec. 31, 2016	Mar. 31, 2016	Dec. 31, 2015
Assets:
Marketable debt securities	$ 4,781.4	$ 5,398.0
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	645.1	2,039.6
Marketable equity securities	18.7	24.9
Derivative contracts	44.5	61.0
Plan assets for deferred compensation	31.0	34.5
Total	5,520.7	7,558.0
Liabilities:
Derivative contracts	13.6	13.6
Contingent consideration obligations	470.9	467.6	$ 508.3	$ 506.0
Total	484.5	481.2
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,500.7	2,663.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,738.2	2,172.5
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	542.5	561.7
Quoted Prices in Active Markets, Level 1 | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0	0.0
Marketable equity securities	18.7	24.9
Derivative contracts	0.0	0.0
Plan assets for deferred compensation	0.0	0.0
Total	18.7	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	645.1	2,039.6
Marketable equity securities	0.0	0.0
Derivative contracts	44.5	61.0
Plan assets for deferred compensation	31.0	34.5
Total	5,502.0	7,533.1
Liabilities:
Derivative contracts	13.6	13.6
Contingent consideration obligations	0.0	0.0
Total	13.6	13.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,500.7	2,663.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,738.2	2,172.5
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	542.5	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	470.9	467.6
Total	470.9	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