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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Sep. 30, 2019
Debt securities, available for sale:
Amortized Cost	$ 210.7	$ 1,033.9
Gross Unrealized Gains	0.0	0.5
Gross Unrealized Losses	(0.1)	(0.1)
Estimated Market Value	210.6	1,034.3
Total marketable securities
Cost or Amortized Cost	212.7
Gross Unrealized Gains	36.0
Gross Unrealized Losses	(0.1)
Estimated Market Value	248.6	1,034.3
U.S. government agencies
Debt securities, available for sale:
Amortized Cost	173.2	698.6
Gross Unrealized Gains	0.0	0.4
Gross Unrealized Losses	(0.1)	0.0
Estimated Market Value	173.1	699.0
Commercial paper
Debt securities, available for sale:
Amortized Cost	36.2	277.9
Gross Unrealized Gains	0.0	0.1
Gross Unrealized Losses	0.0	(0.1)
Estimated Market Value	36.2	277.9
Corporate debt
Debt securities, available for sale:
Amortized Cost	1.3	57.4
Gross Unrealized Gains	0.0	0.0
Gross Unrealized Losses	0.0	0.0
Estimated Market Value	1.3	$ 57.4
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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