Fair Value Disclosures (Tables)

3 Months Ended

Mar. 31, 2013

Fair Value Disclosures [Abstract]

Cash, Cash Equivalents and Investments, as Well as Hierarchy for Financial Instruments Measured at Fair Value on Recurring Basis

The following table presents the Company’s cash equivalents and short-term investments, as well as the hierarchy for its financial instruments measured at fair value on a recurring basis:

			Fair value measurements using:
	Fair Value		Level 1		Level 2		Level 3
March 31, 2013:
Cash equivalents:
Money market funds	$	46,393	$	46,393		—		—
Short-term investments:
U.S. Treasury bills	$	159,957	$	159,957		—		—
December 31, 2012:
Cash equivalents:
Money market funds	$	48,693	$	48,693		—		—
Short-term investments:
U.S. Treasury bills	$	224,939	$	224,939		—		—

Fair Value of Other Financial Instruments Not Measured on Recurring Basis

In the table below, the Company has presented the fair value of other financial instruments that are not measured at fair value on a recurring basis.

			Fair value measurements using:
	Fair Value		Level 1		Level 2		Level 3
March 31, 2013:
Assets:
Bank deposits (included in “Cash and cash equivalents”)	$	65,131	$	65,131		—		—
Liabilities:
Convertible Notes	$	230,953		—	$	230,953		—
December 31, 2012:
Assets:
Bank deposits (included in “Cash and cash equivalents”)	$	22,608	$	22,608		—		—
Liabilities:
Convertible Notes	$	234,813		—	$	234,813		—