Fair Value Measurements (Details) - USD ($) $ in Millions	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017
Assets:
Marketable debt securities	$ 2,538.1		$ 3,689.3
Fair Value, Measurements, Recurring
Assets:
Cash equivalents	1,120.4		705.5
Marketable equity securities	510.0		615.4
Derivative contracts	88.2		66.9
Plan assets for deferred compensation	29.5		25.4
Total	4,286.2		5,102.5
Liabilities:
Derivative contracts	17.5		24.6
Contingent consideration obligations	401.3	$ 421.3	409.8	$ 499.9	$ 498.0	$ 523.6
Total	418.8		434.4
Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,687.7		2,459.2
Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	604.4		969.6
Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	246.0		260.5
Quoted Prices in Active Markets (Level 1) | Fair Value, Measurements, Recurring
Assets:
Cash equivalents	0.0		0.0
Marketable equity securities	176.6		51.7
Derivative contracts	0.0		0.0
Plan assets for deferred compensation	0.0		0.0
Total	176.6		51.7
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	1,120.4		705.5
Marketable equity securities	333.4		563.7
Derivative contracts	88.2		66.9
Plan assets for deferred compensation	29.5		25.4
Total	4,109.6		5,050.8
Liabilities:
Derivative contracts	17.5		24.6
Contingent consideration obligations	0.0		0.0
Total	17.5		24.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Corporate debt securities
Assets:
Marketable debt securities	1,687.7		2,459.2
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Government securities
Assets:
Marketable debt securities	604.4		969.6
Significant Other Observable Inputs (Level 2) | Fair Value, Measurements, Recurring | Mortgage and other asset backed securities
Assets:
Marketable debt securities	246.0		260.5
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	401.3		409.8
Total	401.3		409.8
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