Short-Term Investments (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities
Cost	$ 69,608	$ 71,334
Gross unrealized gains	23	18
Gross unrealized losses	23	14
Fair value	69,608	71,338
U.S. Treasury
Schedule of Available-for-sale Securities
Cost	24,741	29,000
Gross unrealized gains	6	8
Gross unrealized losses	1
Fair value	24,746	29,008
Corporate bonds
Schedule of Available-for-sale Securities
Cost	44,867	42,334
Gross unrealized gains	17	10
Gross unrealized losses	22	14
Fair value	44,862	42,330
Maturities less than one year
Schedule of Available-for-sale Securities
Cost	45,618	34,553
Gross unrealized gains	14	10
Gross unrealized losses	3	3
Fair value	45,629	34,560
Maturities greater than one year
Schedule of Available-for-sale Securities
Cost	23,990	36,781
Gross unrealized gains	9	8
Gross unrealized losses	20	11
Fair value	$ 23,979	$ 36,778