Short-Term Investments - Summary of Available-for-sale Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	$ 770,000	$ 269,236
Gross Unrealized Gains	2	5
Gross Unrealized Losses	(2,227)	(446)
Estimated Fair Value	767,775	268,795
Cash and cash equivalents	402,102	146,363	$ 173,918	$ 60,200
Total short-term investments	767,775	255,845
Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	19,160	5,000
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(36)	(6)
Estimated Fair Value	19,124	4,994
Total short-term investments	19,124	4,994
Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	448,688	143,215
Gross Unrealized Gains	0	4
Gross Unrealized Losses	(1,421)	(235)
Estimated Fair Value	447,267	142,984
Total short-term investments	447,267	142,984
U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Amortized Cost	302,152	121,021
Gross Unrealized Gains	2	1
Gross Unrealized Losses	(770)	(205)
Estimated Fair Value	301,384	120,817
Total short-term investments	301,384	107,867
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	0	12,950
Short-term investments | Certificates of deposit
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	0	0
Short-term investments | Corporate notes and bonds
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	0	0
Short-term investments | U.S. Government agencies
Schedule of Available-for-sale Securities [Line Items]
Cash and cash equivalents	$ 0	$ 12,950