Marketable Securities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Bond	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities, net of tax	$ (682,000)	$ 686,000	$ (719,000)
Available for sale securities remaining maturities greater than three years	0	0
Unrealized gains (losses) on available-for-sale securities, before tax	(200,000)	1,100,000	(700,000)
Unrealized gains (losses) on available-for-sale securities, income tax expense	0	400,000	0
Net realized gains or losses available for sale securities	479,000
Aggregate fair value of securities in an unrealized loss position for less than twelve months	109,200,000	102,500,000
Aggregate fair value of securities in an unrealized loss position for more than twelve months	17,100,000
Aggregate unrealized loss of securities in an unrealized loss position for more than twelve months		0
Other-than-temporary impaired	$ 0	0
Number of investment grade corporate bonds | Bond	46
Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Aggregate unrealized loss of securities in an unrealized loss position for more than twelve months	$ 100,000
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Unrealized gain (loss) on available-for-sale securities, net of tax	600,000
Common Equity Securities [Member] | Audentes Therapeutics, Inc. [Member] | Investment Income
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities	$ 500,000
Common Equity Securities [Member] | Audentes Therapeutics, Inc. [Member] | Investment Income | Maximum [Member]
Schedule Of Available For Sale Securities [Line Items]
Net realized gains or losses available for sale securities		$ (100,000)	$ 0