Debt Security Investments (Tables)

9 Months Ended

Sep. 30, 2012

Debt Security Investments [Abstract]

Amortized cost, gross unrealized gains, gross unrealized losses and the fair value of debt security investments

	September 30, 2012									December 31, 2011
	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value		Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury securities	$	76,009	$	29	$	(8	)	$	76,030	$	36,950	$	—	$	(6	)	$	36,944

Gross unrealized losses and fair value of all available for sale debt security investments

	Less than 12 months					12 months or more				Total
	Fair value		Unrealized losses			Fair value		Unrealized losses		Fair value		Unrealized losses
September 30, 2012
U.S. Treasury securities	$	47,235	$	(8	)	$	—	$	—	$	47,235	$	(8	)
December 31, 2011
U.S. Treasury securities	$	36,944	$	(6	)	$	—	$	—	$	36,944	$	(6	)

Amortized cost and the fair value of debt security investments, by contractual maturity

	September 30, 2012
	Amortized Cost		Fair Value
Due in one year or less	$	56,585	$	56,583
Due after one year through five years		19,424		19,447
Total debt security investments	$	76,009	$	76,030