Marketable Securities - Amortized Cost and Fair Value of Marketable Securities Classified by Stated Maturity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 189,997	$ 112,458
Fair Value	189,876	112,544
Due in 1 year or less
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	62,295	59,384
Fair Value	62,305	59,448
Due in 1 - 2 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	53,133	21,628
Fair Value	53,116	21,641
Due in 2 - 3 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	36,349	10,063
Fair Value	36,300	10,060
Due in 3 - 4 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,958	0
Fair Value	10,961	0
Due in 4 - 9 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,404	7,587
Fair Value	10,402	7,610
Due in 9 - 20 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	1,350	1,629
Fair Value	1,361	1,639
Due in 20 - 33 years
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,508	12,167
Fair Value	$ 15,431	$ 12,146