AVAILABLE-FOR-SALE SECURITIES AND FAIR VALUE MEASUREMENTS - Fair Value Measurements (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Recurring basis
Assets
Total assets measured at estimated fair value	$ 148,813	$ 176,776
Recurring basis | U.S. government agencies
Assets
Total assets measured at estimated fair value	12,429	14,405
Recurring basis | U.S. corporate notes
Assets
Total assets measured at estimated fair value		2,701
Recurring basis | U.S. commercial paper
Assets
Total assets measured at estimated fair value	72,065	10,997
Recurring basis | Money market funds
Assets
Total assets measured at estimated fair value	64,319	148,673
Recurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1
Assets
Total assets measured at estimated fair value	64,319	148,673
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. corporate notes
Assets
Total assets measured at estimated fair value		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	64,319	148,673
Recurring basis | Significant Other Observable Inputs, Level 2
Assets
Total assets measured at estimated fair value	84,494	28,103
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. government agencies
Assets
Total assets measured at estimated fair value	12,429	14,405
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. corporate notes
Assets
Total assets measured at estimated fair value		2,701
Recurring basis | Significant Other Observable Inputs, Level 2 | U.S. commercial paper
Assets
Total assets measured at estimated fair value	72,065	10,997
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. corporate notes
Assets
Total assets measured at estimated fair value		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	689,314	660,070
Nonrecurring basis | Convertible subordinated notes due 2023
Liabilities
Convertible subordinated notes due 2023	202,125	189,100
Nonrecurring basis | Non-Recourse Notes Payable due 2029
Liabilities
Non-recourse notes due 2029	487,189	470,970
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
Convertible subordinated notes due 2023	0	0
Nonrecurring basis | Quoted Prices in Active Markets for Identical Assets, Level 1 | Non-Recourse Notes Payable due 2029
Liabilities
Non-recourse notes due 2029	0	0
Nonrecurring basis | Significant Other Observable Inputs, Level 2
Liabilities
Total fair value of liabilities	689,314	660,070
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Convertible subordinated notes due 2023
Liabilities
Convertible subordinated notes due 2023	202,125	189,100
Nonrecurring basis | Significant Other Observable Inputs, Level 2 | Non-Recourse Notes Payable due 2029
Liabilities
Non-recourse notes due 2029	487,189	470,970
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
Convertible subordinated notes due 2023	0	0
Nonrecurring basis | Significant Unobservable Inputs, Level 3 | Non-Recourse Notes Payable due 2029
Liabilities
Non-recourse notes due 2029	$ 0	$ 0