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Balance Sheet Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Debt securities, available for sale:
Debt securities, available-for-sale, amortized cost	$ 210.7	$ 107.3
Debt Securities, gross unrealized gains	0.0	0.0
Debt securities, gross unrealized losses	(0.1)	(0.2)
Short-term marketable securities	210.6	107.1
Debt Securities, Available-for-sale and Equity Securities, Amortized Cost	212.7
Total marketable securities, cost or amortized cost	248.6	107.1
Total marketable securities, gross unrealized gains	36.0
Total marketable securities, gross unrealized losses	(0.1)
Debt securities with maturities of less than 12 months	92.7
Debt securities with maturities of more than 12 months	$ 14.4
Debt securities held maximum contractual maturities	16 months
Net gain recognized on equity securities
Net gains recognized during the period on equity securities	$ 80.1	0.0	$ 0.0
Less: Net gains recognized during the period on equity securities sold during the period	(44.1)	0.0	0.0
Unrealized gains recognized during the reporting period on equity securities still held at the reporting date	36.0	0.0	$ 0.0
U.S. government agencies
Debt securities, available for sale:
Debt securities, available-for-sale, amortized cost	173.2	87.5
Debt Securities, gross unrealized gains	0.0	0.0
Debt securities, gross unrealized losses	(0.1)	(0.2)
Short-term marketable securities	173.1	87.3
Commercial paper
Debt securities, available for sale:
Debt securities, available-for-sale, amortized cost	36.2	14.7
Debt Securities, gross unrealized gains	0.0	0.0
Debt securities, gross unrealized losses	0.0	0.0
Short-term marketable securities	36.2	14.7
Corporate debt
Debt securities, available for sale:
Debt securities, available-for-sale, amortized cost	1.3	5.1
Debt Securities, gross unrealized gains	0.0	0.0
Debt securities, gross unrealized losses	0.0	0.0
Short-term marketable securities	1.3	$ 5.1
Equity investment in Tandem Diabetes Care, Inc
Short-Term Marketable Securities
Equity investment in Tandem Diabetes Care, Inc., cost or amortized cost	2.0
Equity investment in Tendem Diabetes Care, Inc., gross unrealized gains	36.0
Equity investment in Tandem Diabetes Care, inc., gross unrealized losses	0.0
Equity Invesment in Tandem Diabetes Care, Inc., estimated market value	$ 38.0

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