Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Assets
Total assets	$ 750,567	$ 646,159
Accrued liabilities
Total liabilities	372	1,040
Money market funds
Assets
Cash equivalents	113,213	148,384
Commercial paper
Assets
Cash equivalents	24,095	18,488
Short-term investments	81,226	46,361
Certificates of deposit
Assets
Short-term investments	2,005	2,005
U.S. treasury securities
Assets
Short-term investments	38,156	18,197
U.S. agency securities
Assets
Short-term investments	336,688	303,450
Corporate debt securities
Assets
Short-term investments	143,224	107,517
Municipal securities
Assets
Short-term investments	9,429	1,611
Foreign currency forward contracts
Assets
Other current assets	2,531	146
Accrued liabilities
Accrued liabilities	372	1,040
Level 1
Assets
Total assets	151,369	166,581
Level 1 | Money market funds
Assets
Cash equivalents	113,213	148,384
Level 1 | U.S. treasury securities
Assets
Short-term investments	38,156	18,197
Level 2
Assets
Total assets	599,198	479,578
Accrued liabilities
Total liabilities	372	1,040
Level 2 | Commercial paper
Assets
Cash equivalents	24,095	18,488
Short-term investments	81,226	46,361
Level 2 | Certificates of deposit
Assets
Short-term investments	2,005	2,005
Level 2 | U.S. agency securities
Assets
Short-term investments	336,688	303,450
Level 2 | Corporate debt securities
Assets
Short-term investments	143,224	107,517
Level 2 | Municipal securities
Assets
Short-term investments	9,429	1,611
Level 2 | Foreign currency forward contracts
Assets
Other current assets	2,531	146
Accrued liabilities
Accrued liabilities	$ 372	$ 1,040