MARKETABLE SECURITIES (Details) - USD ($) $ in Thousands | 6 Months Ended | 12 Months Ended |
Jun. 30, 2019 | Dec. 31, 2018 |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Current | $ 122,637 | $ 199,937 |
Minimum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Probabilityof Payment | 75.00% | |
Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Probabilityof Payment | 100.00% | |
Municipal Bonds [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale | $ 25,446 | 17,570 |
Corporate Debt Securities [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale | 252,044 | 246,215 |
Commercial Paper [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale | 28,129 | 50,194 |
Asset-backed Securities [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale | 168,842 | 140,181 |
Government, federal agency, and other sovereign obligations [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale | $ 17,841 | $ 8,894 |
Short-term Investments [Member] | Municipal Bonds [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Short-term Investments [Member] | Corporate Debt Securities [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Short-term Investments [Member] | Commercial Paper [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Short-term Investments [Member] | Asset-backed Securities [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Short-term Investments [Member] | Government, federal agency, and other sovereign obligations [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Other Long-term Investments [Member] | Municipal Bonds [Member] | Minimum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Other Long-term Investments [Member] | Municipal Bonds [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 3 years | 2 years |
Other Long-term Investments [Member] | Corporate Debt Securities [Member] | Minimum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Other Long-term Investments [Member] | Corporate Debt Securities [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 3 years | 3 years |
Other Long-term Investments [Member] | Asset-backed Securities [Member] | Minimum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Other Long-term Investments [Member] | Asset-backed Securities [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 3 years | 3 years |
Other Long-term Investments [Member] | Government, federal agency, and other sovereign obligations [Member] | Minimum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 1 year | 1 year |
Other Long-term Investments [Member] | Government, federal agency, and other sovereign obligations [Member] | Maximum [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Contractual Maturity | 2 years | 3 years |
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | $ 122,422 | $ 200,210 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 220 | 3 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (5) | (276) |
Debt Securities, Available-for-sale | | 199,937 |
Debt Securities, Available-for-sale, Current | 122,637 | |
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Municipal Bonds [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 4,016 | 14,923 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2 | |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | | (25) |
Debt Securities, Available-for-sale | 4,018 | 14,898 |
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Corporate Debt Securities [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 68,319 | 118,823 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 195 | |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | (185) |
Debt Securities, Available-for-sale | 68,514 | 118,638 |
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Commercial Paper [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 28,119 | 50,202 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 10 | 3 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | 0 | (11) |
Debt Securities, Available-for-sale | 28,129 | 50,194 |
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Asset-backed Securities [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 12,040 | 11,765 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 3 | |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (5) | (54) |
Debt Securities, Available-for-sale | 12,038 | 11,711 |
Fair Value, Measurements, Recurring [Member] | Short-term Investments [Member] | Government, federal agency, and other sovereign obligations [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 9,928 | 4,497 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 10 | |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | | (1) |
Debt Securities, Available-for-sale | 9,938 | 4,496 |
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 365,152 | 263,060 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 4,517 | 458 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (4) | (401) |
Debt Securities, Available-for-sale | 369,665 | 263,117 |
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Municipal Bonds [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 21,232 | 2,676 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 196 | |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | | (4) |
Debt Securities, Available-for-sale | 21,428 | 2,672 |
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Corporate Debt Securities [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 181,150 | 127,676 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 2,382 | 196 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (2) | (295) |
Debt Securities, Available-for-sale | 183,530 | 127,577 |
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Asset-backed Securities [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 154,940 | 128,297 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 1,866 | 262 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | (2) | (89) |
Debt Securities, Available-for-sale | 156,804 | 128,470 |
Fair Value, Measurements, Recurring [Member] | Other Long-term Investments [Member] | Government, federal agency, and other sovereign obligations [Member] | | |
Debt Securities, Available-for-sale [Line Items] | | |
Debt Securities, Available-for-sale, Amortized Cost | 7,830 | 4,411 |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain, before Tax | 73 | |
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Loss, before Tax | | (13) |
Debt Securities, Available-for-sale | $ 7,903 | $ 4,398 |