Investment Securities Gross Gains and Losses (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2012	Dec. 31, 2011
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 140,666	$ 31,163
Gross unrealized gains	108	53
Gross unrealized losses	(26)	(6)
Total fixed income securities	140,748	31,210
Corporate bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	40,416	16,307
Gross unrealized gains	49	27
Gross unrealized losses	(5)	(1)
Total fixed income securities	40,460	16,333
Commercial paper [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	8,832	4,998
Gross unrealized gains	2	1
Gross unrealized losses	0	0
Total fixed income securities	8,834	4,999
Negotiable certificate of deposit [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	3,500	3,500
Gross unrealized gains	0	0
Gross unrealized losses	0	0
Total fixed income securities	3,500	3,500
US treasury notes [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	39,661
Gross unrealized gains	1
Gross unrealized losses	(14)
Total fixed income securities	39,648
Agency securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	34,989	3,979
Gross unrealized gains	20	12
Gross unrealized losses	(7)	(4)
Total fixed income securities	35,002	3,987
Municipal bonds [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	4,804	2,379
Gross unrealized gains	32	13
Gross unrealized losses	0	(1)
Total fixed income securities	4,836	2,391
Asset-backed securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	8,464
Gross unrealized gains	4
Gross unrealized losses	0
Total fixed income securities	$ 8,468