Marketable Securities - Amortized Cost and Fair Value of Marketable Securities Classified by Stated Maturity (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 69,023	$ 164,845
Fair Value	69,003	164,619
Due in 1 year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	24,988	46,759
Fair Value	24,988	46,754
Due in 1 - 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	21,579	53,698
Fair Value	21,583	53,639
Due in 2 - 3 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,674	30,558
Fair Value	10,673	30,505
Due in 3 - 4 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,467	11,277
Fair Value	3,463	11,275
Due in 4 - 9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,497	7,280
Fair Value	3,497	7,275
Due in 9 - 20 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,460	1,257
Fair Value	1,464	1,266
Due in 20 - 35 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	3,358	14,016
Fair Value	$ 3,335	$ 13,905