Available-for-Sale Securities and Fair Value Measurements - Fair Value Measurements (Details) - USD ($) $ in Thousands	Mar. 31, 2017	Dec. 31, 2016
Recurring basis
Assets
Total assets measured at estimated fair value	$ 168,825	$ 148,813
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value	6,005	12,429
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	62,878	72,065
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	99,942	64,319
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	99,942	64,319
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. government agencies
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Money market funds
Assets
Total assets measured at estimated fair value	99,942	64,319
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	68,883	84,494
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value	6,005	12,429
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	62,878	72,065
Recurring basis | Significant Other Observable Inputs, Level 2 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. government agencies
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	0	0
Recurring basis | Significant Unobservable Inputs, Level 3 | Money market funds
Assets
Total assets measured at estimated fair value	0	0
Nonrecurring basis
Liabilities
Total fair value of liabilities	715,075	689,314
Nonrecurring basis | Convertible subordinated notes due 2023
Liabilities
2023 Notes	227,248	202,125
Nonrecurring basis | Non-recourse notes due 2029
Liabilities
2029 Notes	487,827	487,189
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
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
Liabilities
2023 Notes	0	0
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Non-recourse notes due 2029
Liabilities
2029 Notes	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2
Liabilities
Total fair value of liabilities	715,075	689,314
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Convertible subordinated notes due 2023
Liabilities
2023 Notes	227,248	202,125
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Non-recourse notes due 2029
Liabilities
2029 Notes	487,827	487,189
Nonrecurring basis | Significant Unobservable Inputs, Level 3
Liabilities
Total fair value of liabilities	0	0
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Convertible subordinated notes due 2023
Liabilities
2023 Notes	0	0
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Non-recourse notes due 2029
Liabilities
2029 Notes	$ 0	$ 0