Marketable Securities - Summary of Company Marketable Securities (Detail) - USD ($)	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	$ 172,822,000	$ 162,970,000
Unrealized Gains	417,000	40,000
Unrealized Losses	(142,000)	(759,000)
Fair Value	173,097,000	162,251,000	$ 0
Common Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	3,000	3,000
Unrealized Gains	24,000	36,000
Fair Value	27,000	39,000
Corporate Bonds [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	162,821,000	157,977,000
Unrealized Gains	393,000	4,000
Unrealized Losses	(142,000)	(759,000)
Fair Value	163,072,000	157,222,000
Commercial Paper [Member]
Schedule Of Available For Sale Securities [Line Items]
Amortized Cost	9,998,000	4,990,000
Fair Value	$ 9,998,000	$ 4,990,000