Fair Value Measurements (Details) (USD $)	0 Months Ended	12 Months Ended
	Jul. 30, 2014	Dec. 31, 2014	Oct. 31, 2014 multiplier	Dec. 31, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase in notes receivable	$ 4,000,000
Multiplier of payment received		2	2
U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. government notes		209,426,000
Fair Value, Measurements, Recurring
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable				4,000,000	[1]
Total financial assets		313,642,000		55,076,000
Fair Value, Measurements, Recurring | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		104,216,000		47,036,000
Other assets - Restricted cash				4,040,000
Fair Value, Measurements, Recurring | Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable				0	[1]
Total financial assets		313,642,000		51,076,000
Fair Value, Measurements, Recurring | Level 1 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		104,216,000		47,036,000
Other assets - Restricted cash				4,040,000
Fair Value, Measurements, Recurring | Level 1 | U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. government notes		209,426,000
Fair Value, Measurements, Recurring | Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable				0	[1]
Total financial assets		0		0
Fair Value, Measurements, Recurring | Level 2 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		0		0
Other assets - Restricted cash				0
Fair Value, Measurements, Recurring | Level 2 | U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. government notes		0
Fair Value, Measurements, Recurring | Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notes receivable				4,000,000	[1]
Total financial assets		0		4,000,000
Fair Value, Measurements, Recurring | Level 3 | Money market funds
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash and cash equivalents		0		0
Other assets - Restricted cash				0
Fair Value, Measurements, Recurring | Level 3 | U.S. government notes
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
U.S. government notes		0
6% Convertible Notes Receivable
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Increase in notes receivable		$ 4,000,000

[1] There were no changes in the fair value of the Level III assets during the year ended December 31, 2013. In 2014, we increased the fair value of our Notes receivable by $4.0 million (See “Note 4 – Investments”). During the year ended December 31, 2014 and 2013, we had no transfers between levels of the fair value hierarchy of its assets measured at fair value. In October 2014, we received payment in full from the borrower equal to two times our outstanding principal balance as well as the unpaid accrued interest through the payment date.