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Marketable Securities (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | $ 2,949,502 | $ 2,434,145 |
Unrealized Gain | 1,049 | 819 |
Unrealized Loss | (4,793) | (2,769) |
Debt securities due in one year or less | 1,722,553 | 1,388,537 |
Debt securities due after one year through three years | 851,547 | 843,605 |
Total debt securities | $ 2,574,100 | $ 2,232,142 |
Realized investment gains (losses), description | Realized gains and losses on sales of investments were not material in 2017, 2016 and 2015. | Realized gains and losses on sales of investments were not material in 2017, 2016 and 2015. | Realized gains and losses on sales of investments were not material in 2017, 2016 and 2015.
Cash Equivalents [Member] |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | $ 194,377 | $ 124,793 |
Unrealized Gain |  | 1 |
Unrealized Loss | (1) |  |
Fair Value | 194,376 | 124,794 |
Investments [Member] |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | 2,755,125 | 2,309,352 |
Unrealized Gain | 1,049 | 818 |
Unrealized Loss | (4,792) | (2,769) |
Fair Value | 2,751,382 | 2,307,401 |
Fair Value Measurements, Recurring |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Fair Value | 2,945,758 | 2,432,195 |
US Treasury Securities |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | 593,599 | 570,695 |
Unrealized Gain | 82 | 253 |
Unrealized Loss | (1,693) | (635) |
US Treasury Securities | Fair Value Measurements, Recurring |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Fair Value | 591,988 | 570,313 |
Foreign Government Debt Securities |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | 6,982 | 17,999 |
Unrealized Loss | (30) | (8) |
Foreign Government Debt Securities | Fair Value Measurements, Recurring |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Fair Value | 6,952 | 17,991 |
Corporate Debt Securities |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | 1,977,329 | 1,645,468 |
Unrealized Gain | 897 | 496 |
Unrealized Loss | (3,066) | (2,126) |
Corporate Debt Securities | Fair Value Measurements, Recurring |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Fair Value | 1,975,160 | 1,643,838 |
Time Deposits |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | 371,515 | 199,906 |
Unrealized Loss | (4) |  |
Time Deposits | Fair Value Measurements, Recurring |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Fair Value | 371,511 | 199,906 |
Equity Securities |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Amortized Cost | 77 | 77 |
Unrealized Gain | 70 | 70 |
Equity Securities | Fair Value Measurements, Recurring |  |  |
Schedule of Available-for-Sale Securities [Line Items] |  |  |
Fair Value | $ 147 | $ 147 |

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