Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016
Assets:
Marketable securities	$ 3,134.2		$ 5,172.5
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	732.2		1,229.4
Marketable equity securities	489.7		11.8
Derivative contracts	40.0		2.7
Plan assets for deferred compensation	28.2		28.5
Total	4,424.3		6,444.9
Liabilities:
Derivative contracts	43.5		111.3
Contingent consideration obligations	499.9	$ 498.0	523.6	$ 492.1	$ 470.9	$ 467.6
Total	543.4		634.9
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable securities	2,045.5		2,609.8
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable securities	828.7		1,919.3
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable securities	260.0		643.4
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0		0.0
Marketable equity securities	70.1		11.8
Derivative contracts	0.0		0.0
Plan assets for deferred compensation	0.0		0.0
Total	70.1		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 securities	0.0		0.0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable securities	0.0		0.0
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable securities	0.0		0.0
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	732.2		1,229.4
Marketable equity securities	419.6		0.0
Derivative contracts	40.0		2.7
Plan assets for deferred compensation	28.2		28.5
Total	4,354.2		6,433.1
Liabilities:
Derivative contracts	43.5		111.3
Contingent consideration obligations	0.0		0.0
Total	43.5		111.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable securities	2,045.5		2,609.8
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable securities	828.7		1,919.3
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable securities	260.0		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	499.9		523.6
Total	499.9		523.6
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable securities	0.0		0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable securities	0.0		0.0
Significant Unobservable Inputs (Level 3) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable securities	$ 0.0		$ 0.0