Fair Value Measurements (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016
Assets:
Marketable debt securities	$ 3,286.2		$ 5,172.5
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,900.9		1,229.4
Marketable equity securities	624.8		11.8
Derivative contracts	35.1		2.7
Plan assets for deferred compensation	26.4		28.5
Total	5,873.4		6,444.9
Liabilities:
Derivative contracts	31.2		111.3
Contingent consideration obligations	412.0	$ 499.9	523.6	$ 522.1	$ 492.1	$ 467.6
Total	443.2		634.9
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,951.6		2,609.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,060.9		1,919.3
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	273.7		643.4
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0		0.0
Marketable equity securities	95.6		11.8
Derivative contracts	0.0		0.0
Plan assets for deferred compensation	0.0		0.0
Total	95.6		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	1,900.9		1,229.4
Marketable equity securities	529.2		0.0
Derivative contracts	35.1		2.7
Plan assets for deferred compensation	26.4		28.5
Total	5,777.8		6,433.1
Liabilities:
Derivative contracts	31.2		111.3
Contingent consideration obligations	0.0		0.0
Total	31.2		111.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,951.6		2,609.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	1,060.9		1,919.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	273.7		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	412.0		523.6
Total	412.0		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