Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Assets
Total assets	$ 2,128,869	$ 646,159
Accrued liabilities
Total liabilities	1,994	1,040
Money market funds
Assets
Cash equivalents	884,275	148,384
Commercial paper
Assets
Cash equivalents	16,897	18,488
Short-term investments	76,738	46,361
Certificates of deposit
Assets
Short-term investments	2,004	2,005
U.S. treasury securities
Assets
Cash equivalents	351,697
Short-term investments	92,731	18,197
U.S. agency securities
Assets
Short-term investments	432,779	303,450
Corporate debt securities
Assets
Short-term investments	257,432	107,517
Municipal securities
Assets
Short-term investments	14,309	1,611
Foreign currency derivative contracts
Assets
Other current assets	7	146
Accrued liabilities
Accrued liabilities	1,994	1,040
Level 1
Assets
Total assets	1,328,703	166,581
Level 1 | Money market funds
Assets
Cash equivalents	884,275	148,384
Level 1 | U.S. treasury securities
Assets
Cash equivalents	351,697
Short-term investments	92,731	18,197
Level 2
Assets
Total assets	800,166	479,578
Accrued liabilities
Total liabilities	1,994	1,040
Level 2 | Commercial paper
Assets
Cash equivalents	16,897	18,488
Short-term investments	76,738	46,361
Level 2 | Certificates of deposit
Assets
Short-term investments	2,004	2,005
Level 2 | U.S. agency securities
Assets
Short-term investments	432,779	303,450
Level 2 | Corporate debt securities
Assets
Short-term investments	257,432	107,517
Level 2 | Municipal securities
Assets
Short-term investments	14,309	1,611
Level 2 | Foreign currency derivative contracts
Assets
Other current assets	7	146
Accrued liabilities
Accrued liabilities	$ 1,994	$ 1,040