Financial Instruments - Schedule of Cash, Cash Equivalents and Marketable Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 617,053	$ 585,909
Unrealized Gains	59	31
Unrealized Losses	(1,152)	(207)
Fair Value	615,960	585,733
Cash and cash equivalents	200,058	367,515	478,241	183,532
Short-Term Marketable Securities	196,037	118,970
Long-Term Marketable Securities	219,865	99,248
Cash [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost, Cash	154,775	141,460
Unrealized Gains, Cash
Unrealized Losses, Cash
Fair Value, Cash	154,775	141,460
Cash and Cash Equivalents, Cash	154,775	141,460
Short-Term Marketable Securities, Cash
Long-Term Marketable Securities, Cash
Level 1 [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	10,963	215,052
Unrealized Gains
Unrealized Losses
Fair Value	10,963	215,052
Cash and cash equivalents	10,963	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	10,963	215,052
Unrealized Gains
Unrealized Losses
Fair Value	10,963	215,052
Cash and cash equivalents	10,963	215,052
Short-Term Marketable Securities
Long-Term Marketable Securities
Level 2 [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	451,315	229,397
Unrealized Gains	59	31
Unrealized Losses	(1,152)	(207)
Fair Value	450,222	229,221
Cash and cash equivalents	34,320	11,003
Short-Term Marketable Securities	196,037	118,970
Long-Term Marketable Securities	219,865	99,248
Level 2 [Member] | U.S. agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	79,048	13,634
Unrealized Gains	11	4
Unrealized Losses	(126)	(3)
Fair Value	78,933	13,635
Cash and cash equivalents
Short-Term Marketable Securities	29,375	7,635
Long-Term Marketable Securities	49,558	6,000
Level 2 [Member] | U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	49,616
Unrealized Gains
Unrealized Losses	(1)
Fair Value	49,615
Cash and cash equivalents	34,320
Short-Term Marketable Securities	15,295
Long-Term Marketable Securities
Level 2 [Member] | Certificates of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	23,701
Unrealized Gains
Unrealized Losses
Fair Value	23,729
Cash and cash equivalents
Short-Term Marketable Securities	21,127
Long-Term Marketable Securities	2,602
Level 2 [Member] | Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	15,492	48,710
Unrealized Gains	4	15
Unrealized Losses		(22)
Fair Value	15,496	48,703
Cash and cash equivalents		9,999
Short-Term Marketable Securities	15,496	38,704
Long-Term Marketable Securities
Level 2 [Member] | Corporate securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	278,457	162,050
Unrealized Gains	16	12
Unrealized Losses	(1,023)	(180)
Fair Value	277,450	161,882
Cash and cash equivalents		1,004
Short-Term Marketable Securities	109,745	67,630
Long-Term Marketable Securities	167,705	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)	(2)
Fair Value	4,999	5,001
Cash and cash equivalents
Short-Term Marketable Securities	4,999	5,001
Long-Term Marketable Securities