Marketable Securities - Amortized Cost and Fair Value of Marketable Securities Classified by Stated Maturity (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 51,007	$ 164,845
Fair Value	51,009	164,619
Due in 1 year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	17,783	46,759
Fair Value	17,870	46,754
Due in 1 - 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,900	53,698
Fair Value	15,858	53,639
Due in 2 - 3 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	7,959	30,558
Fair Value	7,934	30,505
Due in 3 - 4 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,399	11,277
Fair Value	2,408	11,275
Due in 4 - 9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,844	7,280
Fair Value	2,843	7,275
Due in 9 - 20 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,397	1,257
Fair Value	1,394	1,266
Due in 20 - 35 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	2,725	14,016
Fair Value	$ 2,702	$ 13,905