Available-for-Sale Securities and Fair Value Measurements - Schedule of estimated fair values (Details) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	$ 151,149	$ 176,776
U.S. government agencies
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	19,976	14,405
U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value		2,701
U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	52,535	10,997
Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	78,638	148,673
Recurring basis
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	151,149	176,776
Recurring basis | U.S. government agencies
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	19,976	14,405
Recurring basis | U.S. corporate notes
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value		2,701
Recurring basis | U.S. commercial paper
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	52,535	10,997
Recurring basis | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	78,638	148,673
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	78,638	148,673
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government agencies
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
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
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	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	78,638	148,673
Recurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	72,511	28,103
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	19,976	14,405
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		2,701
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	52,535	10,997
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
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agencies
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
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 | Money market funds
Estimated fair values of entity's financial assets and liabilities
Total assets measured at fair value	0	0
Nonrecurring basis
Estimated fair values of entity's financial assets and liabilities
Total fair value of liabilities	691,842	660,070
Nonrecurring basis | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	197,825	189,100
Nonrecurring basis | 9% fixed rate term notes due 2029
Estimated fair values of entity's financial assets and liabilities
Non-recourse notes due 2029	494,017	470,970
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Estimated fair values of entity's financial assets and liabilities
Total fair value of liabilities	0	0
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	0	0
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | 9% fixed rate term notes due 2029
Estimated fair values of entity's financial assets and liabilities
Non-recourse notes due 2029	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2
Estimated fair values of entity's financial assets and liabilities
Total fair value of liabilities	691,842	660,070
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	197,825	189,100
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | 9% fixed rate term notes due 2029
Estimated fair values of entity's financial assets and liabilities
Non-recourse notes due 2029	494,017	470,970
Nonrecurring basis | Significant Unobservable Inputs, Level 3
Estimated fair values of entity's financial assets and liabilities
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Convertible Subordinated Notes Due 2023
Estimated fair values of entity's financial assets and liabilities
Convertible subordinated notes due 2023	0	0
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | 9% fixed rate term notes due 2029
Estimated fair values of entity's financial assets and liabilities
Non-recourse notes due 2029	$ 0	$ 0