Cash, Cash Equivalents and Available-for-Sale Securities (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Cash and cash equivalents and available-for-sale securities, Amortized Cost
Available-for-sale securities including cash and cash equivalents, Amortized Cost	$ 343,221	$ 214,553
Less amounts classified as cash equivalents, Amortized Cost	(103,262)	(58,605)	(39,554)	(37,216)
Available-for-sale securities, Amortized Cost	239,959	155,948
Available-for-sale securities, Gross Unrealized Gains	69	33
Available-for-sale securities, Gross Unrealized Losses	(4)
Cash and cash equivalents and available-for-sale securities, Estimated Fair Value
Available-for-sale securities including cash and cash equivalents, Estimated Fair Value	343,286	214,586
Less amounts classified as cash equivalents, Estimated Fair Value	(103,262)	(58,605)
Available-for-sale securities, Estimated Fair Value	240,024	155,981
Maximum original contractual maturities of available for sale securities	23 months
Cash and money market funds
Cash and cash equivalents and available-for-sale securities, Amortized Cost
Available-for-sale securities including cash and cash equivalents, Amortized Cost	103,262	58,605
Cash and cash equivalents and available-for-sale securities, Estimated Fair Value
Available-for-sale securities including cash and cash equivalents, Estimated Fair Value	103,262	58,605
U.S. Treasury securities
Cash and cash equivalents and available-for-sale securities, Amortized Cost
Available-for-sale securities including cash and cash equivalents, Amortized Cost	239,959	155,948
Available-for-sale securities, Gross Unrealized Gains	69	33
Available-for-sale securities, Gross Unrealized Losses	(4)
Cash and cash equivalents and available-for-sale securities, Estimated Fair Value
Available-for-sale securities including cash and cash equivalents, Estimated Fair Value	$ 240,024	$ 155,981