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 |