Marketable Securities - Amortized Cost and Fair Value of Marketable Securities Classified by Stated Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 522	$ 51,007
Fair Value	522	51,009
Due in 1 year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	522	17,783
Fair Value	522	17,870
Due in 1-4 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	26,258
Fair Value	0	26,200
Due in 5-9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	2,844
Fair Value	0	2,843
Due in more than 9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	0	4,122
Fair Value	$ 0	$ 4,096