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Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Assets:
Marketable debt securities	$ 5,022.2		$ 5,398.0
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,184.3		2,039.6
Marketable equity securities	11.8		24.9
Derivative contracts	1.6		61.0
Plan assets for deferred compensation	30.1		34.5
Total	6,250.0		7,558.0
Liabilities:
Derivative contracts	106.2		13.6
Contingent consideration obligations	522.1	$ 492.1	467.6	$ 521.6	$ 515.7	$ 506.0
Total	628.3		481.2
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,584.7		2,663.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,870.0		2,172.5
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	567.5		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	1,184.3		2,039.6
Marketable equity securities	0.0		0.0
Derivative contracts	1.6		61.0
Plan assets for deferred compensation	30.1		34.5
Total	6,238.2		7,533.1
Liabilities:
Derivative contracts	106.2		13.6
Contingent consideration obligations	0.0		0.0
Total	106.2		13.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	2,584.7		2,663.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,870.0		2,172.5
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	567.5		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	522.1		467.6
Total	522.1		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

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