Marketable Securities Available-For-Sale - Amortized Cost and Fair Value of Marketable Securities Classified by Stated Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 27,763	$ 51,007
Fair Value	27,771	51,009
Due in 1 year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,372	17,783
Fair Value	17,374	17,870
Due in 1 - 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,277	15,900
Fair Value	5,284	15,858
Due in 2 - 3 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,754	7,959
Fair Value	2,753	7,934
Due in 3 - 4 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	140	2,399
Fair Value	140	2,408
Due in 4 - 9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	917	2,844
Fair Value	919	2,843
Due in 9 - 20 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	931	1,397
Fair Value	929	1,394
Due in 20 - 35 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	372	2,725
Fair Value	$ 372	$ 2,702