Fair Value Measurements (Details) (USD $)	Jun. 30, 2014	Dec. 31, 2013
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	$ 346,600,000	$ 502,831,000
U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	23,553,000	42,158,000
U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	54,722,000	141,331,000
U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	33,628,000	94,977,000
U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	34,942,000	102,022,000
Equity securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	13,925,000	0
Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	185,830,000	122,343,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	223,308,000	327,827,000
Total fair value of liabilities	0	0
Fair value measured transfer to Level 2 From Level 1		5,400,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	23,553,000	42,158,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	98,236,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	61,591,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	3,499,000
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Equity securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	13,925,000	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	185,830,000	122,343,000
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	123,292,000	175,004,000
Fair value measured transfer to Level 2 From Level 1	5,400,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	54,722,000	43,095,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	33,628,000	33,386,000
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	34,942,000	98,523,000
Recurring basis | Significant Other Observable Inputs, Level 2 | Equity securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Total fair value of liabilities	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Equity securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Recurring basis | Total
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	346,600,000	502,831,000
Recurring basis | Total | U.S. government securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	23,553,000	42,158,000
Recurring basis | Total | U.S. government agency securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	54,722,000	141,331,000
Recurring basis | Total | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	33,628,000	94,977,000
Recurring basis | Total | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	34,942,000	102,022,000
Recurring basis | Total | Equity securities
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	13,925,000
Recurring basis | Total | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	185,830,000	122,343,000
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	0	0
Non-recourse notes due 2029	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	417,148,000	408,250,000
Non-recourse notes due 2029	454,500,000
Total fair value of liabilities	871,648,000
Nonrecurring basis | Significant Unobservable Inputs, Level 3
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	0	0
Non-recourse notes due 2029	0
Nonrecurring basis | Total
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	417,148,000	408,250,000
Non-recourse notes due 2029	454,500,000
Total fair value of liabilities	$ 871,648,000