Marketable Securities (Details) - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Marketable securities by security type
Cost	$ 429,568	$ 417,845
Gross unrealized gains	37	72
Gross unrealized losses	(554)	(791)
Estimated fair value	429,051	417,126
Commercial paper and corporate bonds [Member]
Marketable securities by security type
Cost	415,067	417,845
Gross unrealized gains	30	72
Gross unrealized losses	(546)	(791)
Estimated fair value	414,551	$ 417,126
U.S. government agency debt securities [Member]
Marketable securities by security type
Cost	14,501
Gross unrealized gains	7
Gross unrealized losses	(8)
Estimated fair value	$ 14,500