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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Debt securities, available for sale:
Amortized Cost	$ 72.9		$ 210.7
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	0.0		(0.1)
Estimated Market Value	72.9	$ 210.6	210.6
Total marketable securities
Cost or Amortized Cost			212.7
Gross Unrealized Gains			36.0
Gross Unrealized Losses			(0.1)
Estimated Market Value	72.9		248.6
Equity Securities Net Gain
Net gains recognized during the period on equity securities	(4.2)	7.4
U.S. government agencies
Debt securities, available for sale:
Amortized Cost	54.8		173.2
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	0.0		(0.1)
Estimated Market Value	54.8	173.1	173.1
Commercial paper
Debt securities, available for sale:
Amortized Cost	3.0		36.2
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	0.0		0.0
Estimated Market Value	3.0	36.2	36.2
Corporate debt
Debt securities, available for sale:
Amortized Cost	15.1		1.3
Gross Unrealized Gains	0.0		0.0
Gross Unrealized Losses	0.0		0.0
Estimated Market Value	$ 15.1	$ 1.3	1.3
Equity investment in Tandem Diabetes Care, Inc
Equity investment in Tandem Diabetes Care, Inc.
Cost or Amortized Cost			2.0
Gross Unrealized Gains			36.0
Gross Unrealized Losses			0.0
Estimated Market Value			$ 38.0

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