Financial Instruments - Schedule of Cash, Cash Equivalents and Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012	Sep. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 596,133	$ 585,909
Unrealized Gains	114	31
Unrealized Losses	(367)	(207)
Fair Value	595,880	585,733
Cash and cash equivalents	276,358	367,515	548,372	183,532
Short-Term Marketable Securities	155,997	118,970
Long-Term Marketable Securities	163,525	99,248
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,958	215,052
Unrealized Gains
Unrealized Losses
Fair Value	44,958	215,052
Cash and cash equivalents	44,958	215,052
Short-Term Marketable Securities
Long-Term Marketable Securities
Level 1 [Member] | Money Market Funds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	44,958	215,052
Unrealized Gains
Unrealized Losses
Fair Value	44,958	215,052
Cash and cash equivalents	44,958	215,052
Short-Term Marketable Securities
Long-Term Marketable Securities
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	319,775	229,397
Unrealized Gains	114	31
Unrealized Losses	(367)	(207)
Fair Value	319,522	229,221
Cash and cash equivalents		11,003
Short-Term Marketable Securities	155,997	118,970
Long-Term Marketable Securities	163,525	99,248
Level 2 [Member] | U.S. Agency Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	38,522	13,634
Unrealized Gains	13	4
Unrealized Losses	(28)	(3)
Fair Value	38,507	13,635
Cash and cash equivalents
Short-Term Marketable Securities	16,026	7,635
Long-Term Marketable Securities	22,481	6,000
Level 2 [Member] | U.S. Treasury Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,298
Unrealized Gains	1
Unrealized Losses
Fair Value	15,299
Cash and cash equivalents
Short-Term Marketable Securities	15,299
Long-Term Marketable Securities
Level 2 [Member] | Certificates of Deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,200
Unrealized Gains	22
Unrealized Losses
Fair Value	19,222
Cash and cash equivalents
Short-Term Marketable Securities	16,221
Long-Term Marketable Securities	3,001
Level 2 [Member] | Commercial Paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	999	48,710
Unrealized Gains	1	15
Unrealized Losses		(22)
Fair Value	1,000	48,703
Cash and cash equivalents		9,999
Short-Term Marketable Securities	1,000	38,704
Long-Term Marketable Securities
Level 2 [Member] | Corporate Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	240,755	162,050
Unrealized Gains	77	12
Unrealized Losses	(339)	(180)
Fair Value	240,493	161,882
Cash and cash equivalents		1,004
Short-Term Marketable Securities	102,450	67,630
Long-Term Marketable Securities	138,043	93,248
Level 2 [Member] | Municipal Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	5,001	5,003
Unrealized Gains
Unrealized Losses		(2)
Fair Value	5,001	5,001
Cash and cash equivalents
Short-Term Marketable Securities	5,001	5,001
Long-Term Marketable Securities
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash	231,400	141,460
Unrealized Gains, Cash
Unrealized Losses, Cash
Cash and Cash Equivalents, Cash	231,400	141,460
Short-Term Marketable Securities, Cash
Long-Term Marketable Securities, Cash