Marketable Securities - Additional Information (Detail)	3 Months Ended	6 Months Ended	12 Months Ended
Jun. 30, 2017 USD ($) Bond	Jun. 30, 2016 USD ($)	Jun. 30, 2017 USD ($) Bond	Jun. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Available for sale securities remaining maturities greater than three years	$ 0		$ 0		$ 0
Unrealized gains (losses) on available-for-sale securities, before tax	(100,000)	$ 200,000	(100,000)	$ 1,200,000
Unrealized gains (losses) available-for-sale securities, income tax expense	0	0	0	0
Realized gains or losses available for sale securities			480,000
Aggregate fair value of securities in an unrealized loss position for less than twelve months	$ 127,200,000		127,200,000		102,500,000
Other-than-temporary impaired			$ 0		$ 0
Number of investment grade corporate bonds \| Bond	45		45
Audentes Therapeutics, Inc. [Member] \| Common Equity Securities [Member] \| Investment Income [Member]
Schedule Of Available For Sale Securities [Line Items]
Realized gains or losses available for sale securities	$ 0	$ 0	$ 500,000	$ 0

Available for sale securities remaining maturities greater than three years.

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This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized loss position for less than twelve months.

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Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

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Amount before tax and reclassification adjustments of unrealized holding gain (loss) on available-for-sale securities.

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The amount by which the fair value of an investment in debt and equity securities categorized as Available-for-sale is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

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