Financial Statement Details - Short Term Marketable Securities, Available for Sale (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financial Statement Details [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	$ 0.0		$ 0.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0.2		0.2
Available-for-sale Securities, Debt Securities	250.8		250.8
Available-for-sale Debt Securities, Amortized Cost Basis	251.0		251.0
Estimated Market Value	305.9		305.9		$ 107.1
Gross Unrealized Losses	(0.2)		(0.2)		(0.2)
Gross Unrealized Gains	50.1		50.1		0.0
Cost or Amortized Cost	256.0		256.0		107.3
Available-for-sale Equity Securities, Gross Unrealized Gain	42.7	$ 0.0	50.1	$ 0.0
Available-for-sale Securities, Current	249.8		249.8
Available-for-sale securities, noncurrent	1.0		1.0
U.S. government agencies
Financial Statement Details [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0.0		0.0		0.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0.2		0.2		0.2
Available-for-sale Securities, Debt Securities	199.4		199.4		87.3
Available-for-sale Debt Securities, Amortized Cost Basis	199.6		199.6		87.5
Commercial paper
Financial Statement Details [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0.0		0.0		0.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0.0		0.0		0.0
Available-for-sale Securities, Debt Securities	49.3		49.3		5.1
Available-for-sale Debt Securities, Amortized Cost Basis	49.3		49.3		5.1
Corporate debt
Financial Statement Details [Line Items]
Available-for-sale Debt Securities, Accumulated Gross Unrealized Gain, before Tax	0.0		0.0		0.0
Available-for-sale Debt Securities, Accumulated Gross Unrealized Loss, before Tax	0.0		0.0		0.0
Available-for-sale Securities, Debt Securities	2.1		2.1		14.7
Available-for-sale Debt Securities, Amortized Cost Basis	2.1		2.1		$ 14.7
Equity investment in Tandem Diabetes Care, Inc
Financial Statement Details [Line Items]
Available-for-sale Equity Securities, Amortized Cost Basis	5.0		5.0
Available-for-sale Equity Securities, Accumulated Gross Unrealized Gain, before Tax	50.1		50.1
Available-for-sale Equity Securities, Accumulated Gross Unrealized Loss, before Tax	0.0		0.0
Available-for-sale Securities, Equity Securities, Current	$ 55.1		55.1
Available-for-sale Equity Securities, Gross Unrealized Gain			$ 50.1

Financial Statement Details [Line Items]

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Investments in debt and equity securities which are categorized neither as held-to-maturity nor trading and which are intended to be sold or mature more than one year from the balance sheet date or operating cycle, if longer. Such securities are reported at fair value; unrealized gains (losses) related to Available-for-sale Securities are excluded from earnings and reported in a separate component of shareholders' equity (other comprehensive income), unless the Available-for-sale security is designated as a hedge or is determined to have had an other than temporary decline in fair value below its amortized cost basis. All or a portion of the unrealized holding gain (loss) of an Available-for-sale security that is designated as being hedged in a fair value hedge is recognized in earnings during the period of the hedge, as are other than temporary declines in fair value below the cost basis for investments in equity securities and debt securities that an entity intends to sell or it is more likely than not that it will be required to sell before the recovery of its amortized cost basis. Other than temporary declines in fair value below the cost basis for debt securities categorized as Available-for-sale that an entity does not intend to sell and for which it is not more likely than not that the entity will be required to sell before the recovery of its amortized cost basis are bifurcated into credit losses and losses related to all other factors. Other than temporary declines in fair value below cost basis related to credit losses are recognized in earnings, and losses related to all other factors are recognized in other comprehensive income.

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