Marketable Securities (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Marketable securities by security type
Cost	$ 417,607	$ 414,069
Gross unrealized gains	189	125
Gross unrealized losses	(446)	(197)
Estimated fair value	417,350	413,997
Commercial paper and corporate bonds [Member]
Marketable securities by security type
Cost	417,607	378,135
Gross unrealized gains	189	122
Gross unrealized losses	(446)	(196)
Estimated fair value	417,350	378,061
U.S. government agency debt securities [Member]
Marketable securities by security type
Cost		35,934
Gross unrealized gains		3
Gross unrealized losses	0	(1)
Estimated fair value		$ 35,936