Cash and Investments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012	Dec. 31, 2011
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Cash and cash equivalents, amortized cost	$ 264,939	$ 270,408	$ 342,248	$ 339,048
Total cash, cash equivalents, and short-term investments, amortized cost	829,887	749,131
Total cash, cash equivalents, and short-term investments, gross unrealized gains	515	498
Total cash, cash equivalents, and short-term investments, gross unrealized losses	(95)	(80)
Total cash, cash equivalents, and short-term investments, estimated fair market value	830,307	749,549
Commercial paper
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Short-term investments, amortized cost	67,218	46,352
Short-term investments, gross unrealized gains	31	9
Short-term investments, gross unrealized losses	(12)
Short-term investments, estimated fair market value	67,237	46,361
Certificates of deposit
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Short-term investments, amortized cost	2,000	2,000
Short-term investments, gross unrealized gains	7	5
Short-term investments, estimated fair market value	2,007	2,005
U.S. treasury securities
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Short-term investments, amortized cost	31,695	18,184
Short-term investments, gross unrealized gains	17	13
Short-term investments, estimated fair market value	31,712	18,197
Agency securities
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Short-term investments, amortized cost	336,038	302,991
Short-term investments, gross unrealized gains	392	460
Short-term investments, gross unrealized losses	(43)	(1)
Short-term investments, estimated fair market value	336,387	303,450
Corporate debt securities
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Short-term investments, amortized cost	126,392	107,585
Short-term investments, gross unrealized gains	59	10
Short-term investments, gross unrealized losses	(40)	(78)
Short-term investments, estimated fair market value	126,411	107,517
Municipal securities
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Short-term investments, amortized cost	1,610	1,612
Short-term investments, gross unrealized gains	4	0
Short-term investments, gross unrealized losses		(1)
Short-term investments, estimated fair market value	1,614	1,611
Cash
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Cash and cash equivalents, amortized cost	107,321	103,536
Cash and cash equivalents, estimated fair market value	107,321	103,536
Money market funds
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Cash and cash equivalents, amortized cost	126,125	148,384
Cash and cash equivalents, estimated fair market value	126,125	148,384
Corporate debt securities
Cash, Cash Equivalents and Available-for-sale Investments [Line Items]
Cash and cash equivalents, amortized cost	31,488	18,487
Cash and cash equivalents, gross unrealized gain	5	1
Cash and cash equivalents, estimated fair market value	$ 31,493	$ 18,488