Cash and Investments - Unrealized Loss Positions (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	$ 321,726	$ 2,221,231
Less than 12 months, unrealized losses	(130)	(4,382)
12 Months or greater, fair value	9,677	0
12 Months or greater, unrealized losses	(10)	0
Total fair value	331,403	2,221,231
Total unrealized losses	(140)	(4,382)
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	994	28,482
Less than 12 months, unrealized losses	0	(23)
12 Months or greater, fair value	0	0
12 Months or greater, unrealized losses	0	0
Total fair value	994	28,482
Total unrealized losses	0	(23)
U.S. treasury securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	93,320	777,237
Less than 12 months, unrealized losses	(11)	(959)
12 Months or greater, fair value	0	0
12 Months or greater, unrealized losses	0	0
Total fair value	93,320	777,237
Total unrealized losses	(11)	(959)
U.S. agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	60,195	472,236
Less than 12 months, unrealized losses	(9)	(739)
12 Months or greater, fair value	0	0
12 Months or greater, unrealized losses	0	0
Total fair value	60,195	472,236
Total unrealized losses	(9)	(739)
Corporate debt securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, fair value	167,217	943,276
Less than 12 months, unrealized losses	(110)	(2,661)
12 Months or greater, fair value	9,677	0
12 Months or greater, unrealized losses	(10)	0
Total fair value	176,894	943,276
Total unrealized losses	$ (120)	$ (2,661)