Financial Statement Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financial Statement Details [Line Items]
Debt securities, available for sale	$ 258.6		$ 258.6
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	(0.1)		(0.1)
Estimated Market Value	258.5		258.5
Cost or Amortized Cost	260.6		260.6		$ 107.3
Gross Unrealized Gains	40.8		40.8		0.0
Gross Unrealized Losses	(0.1)		(0.1)		(0.2)
Estimated Market Value	301.3		301.3		107.1
Net gains recognized during the period on equity securities	34.9	$ 0.0	85.0	$ 0.0
Less: Net gains recognized during the period on equity securities sold during the period	(14.1)	0.0	(44.2)	0.0
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date	20.8	$ 0.0	40.8	$ 0.0
U.S. government agencies
Financial Statement Details [Line Items]
Debt securities, available for sale	189.7		189.7		87.5
Gross Unrealized Gains	0.0		0.0		0.0
Gross Unrealized Losses	(0.1)		(0.1)		(0.2)
Estimated Market Value	189.6		189.6		87.3
Commercial paper
Financial Statement Details [Line Items]
Debt securities, available for sale	67.9		67.9		5.1
Gross Unrealized Gains	0.0		0.0		0.0
Gross Unrealized Losses	0.0		0.0		0.0
Estimated Market Value	67.9		67.9		5.1
Corporate debt
Financial Statement Details [Line Items]
Debt securities, available for sale	1.0		1.0		14.7
Gross Unrealized Gains	0.0		0.0		0.0
Gross Unrealized Losses	0.0		0.0		0.0
Estimated Market Value	1.0		1.0		$ 14.7
Equity investment in Tandem Diabetes Care, Inc
Financial Statement Details [Line Items]
Equity investment in Tandem Diabetes Care, Inc	2.0		2.0
Gross Unrealized Gains	40.8		40.8
Gross Unrealized Losses	0.0		0.0
Estimated Market Value	$ 42.8		$ 42.8

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