Fair Value Measurements (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2014	Dec. 31, 2013
Estimated fair values
Total assets measured at estimated fair value	$ 374,975	$ 833
Fair value measurements transfer to Level 1 From Level 2	0
Fair value measurements, liabilities transfer from Level 1 to Level 2	0
Fair value measurements, liabilities transfer from Level 2 to Level 1	0
Fair value measured transfer to Level 2 From Level 1	0
U.S. government securities
Estimated fair values
Total assets measured at estimated fair value	26,051
U.S. government agency securities
Estimated fair values
Total assets measured at estimated fair value	54,722
U.S. corporate notes
Estimated fair values
Total assets measured at estimated fair value	38,455
Money market funds
Estimated fair values
Total assets measured at estimated fair value	255,747	833
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values
Total assets measured at estimated fair value	281,798	833
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government securities
Estimated fair values
Total assets measured at estimated fair value	26,051
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government agency securities
Estimated fair values
Total assets measured at estimated fair value	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Estimated fair values
Total assets measured at estimated fair value	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Estimated fair values
Total assets measured at estimated fair value	255,747	833
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values
Total assets measured at estimated fair value	93,177	0
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Estimated fair values
Total assets measured at estimated fair value	0
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agency securities
Estimated fair values
Total assets measured at estimated fair value	54,722
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Estimated fair values
Total assets measured at estimated fair value	38,455
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Estimated fair values
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3
Estimated fair values
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government securities
Estimated fair values
Total assets measured at estimated fair value	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agency securities
Estimated fair values
Total assets measured at estimated fair value	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Estimated fair values
Total assets measured at estimated fair value	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Estimated fair values
Total assets measured at estimated fair value	0	0
Recurring basis | Total
Estimated fair values
Total assets measured at estimated fair value	374,975	833
Recurring basis | Total | U.S. government securities
Estimated fair values
Total assets measured at estimated fair value	26,051
Recurring basis | Total | U.S. government agency securities
Estimated fair values
Total assets measured at estimated fair value	54,722
Recurring basis | Total | U.S. corporate notes
Estimated fair values
Total assets measured at estimated fair value	38,455
Recurring basis | Total | Money market funds
Estimated fair values
Total assets measured at estimated fair value	$ 255,747	$ 833