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Investments, Summary of Investments (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
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Summary of Investments [Abstract] |  |  |  |  |
Cost |  | $ 1,874,529 |  |  |
Gross unrealized gains |  | 556 |  |  |
Gross unrealized losses |  | (4,068) |  |  |
Estimated fair value |  | 1,871,017 |  |  |
Securities with Maturity of One Year or Less [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 1,433,930 |  | $ 643,193 |
Gross unrealized gains |  | 31 |  | 6 |
Gross unrealized losses |  | (2,362) |  | (1,103) |
Estimated fair value |  | 1,431,599 |  | 642,096 |
Securities with Maturity of More than One Year [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 439,387 |  | 268,401 |
Gross unrealized gains |  | 388 |  | 8 |
Gross unrealized losses |  | (1,706) |  | (1,969) |
Estimated fair value |  | 438,069 |  | 266,440 |
Available-for-sale Securities [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 1,873,317 |  | 911,594 |
Gross unrealized gains |  | 419 |  | 14 |
Gross unrealized losses |  | (4,068) |  | (3,072) |
Estimated fair value |  | 1,869,668 |  | 908,536 |
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1],[2] | 956,879 |  | 500,599 |
Gross unrealized gains |  | 13 |  | 2 |
Gross unrealized losses |  | (1,858) |  | (752) |
Estimated fair value |  | 955,034 |  | 499,849 |
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 299,018 |  | 148,663 |
Gross unrealized gains |  | 194 |  | 8 |
Gross unrealized losses |  | (1,286) |  | (1,059) |
Estimated fair value |  | 297,926 |  | 147,612 |
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 168,839 |  | 83,926 |
Gross unrealized gains |  | 3 |  | 0 |
Gross unrealized losses |  | (104) |  | (212) |
Estimated fair value |  | 168,738 |  | 83,714 |
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 107,789 |  | 52,779 |
Gross unrealized gains |  | 194 |  | 0 |
Gross unrealized losses |  | (109) |  | (168) |
Estimated fair value |  | 107,874 |  | 52,611 |
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 244,640 | [2] | 29,428 |
Gross unrealized gains |  | 15 |  | 0 |
Gross unrealized losses |  | (77) |  | (17) |
Estimated fair value |  | 244,578 |  | 29,411 |
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 15,600 |  | 1,409 |
Gross unrealized gains |  | 0 |  | 0 |
Gross unrealized losses |  | (24) |  | (2) |
Estimated fair value |  | 15,576 |  | 1,407 |
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 63,572 |  | 29,240 | [2]
Gross unrealized gains |  | 0 |  | 4 |
Gross unrealized losses |  | (323) |  | (122) |
Estimated fair value |  | 63,249 |  | 29,122 |
Debt Securities issued by States of the U.S. and Political Subdivisions of the States [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [1] | 16,980 |  | 65,550 |
Gross unrealized gains |  | 0 |  | 0 |
Gross unrealized losses |  | (287) |  | (740) |
Estimated fair value |  | 16,693 |  | $ 64,810 |
Equity Securities [Member] |  |  |  |  |
Summary of Investments [Abstract] |  |  |  |  |
Cost | [3] | 1,212 |  |  |
Gross unrealized gains |  | 137 |  |  |
Gross unrealized losses |  | 0 |  |  |
Estimated fair value |  | $ 1,349 |  |  |
[1] | We hold our available-for-sale securities at amortized cost.
[2] | Includes investments classified as cash equivalents on our consolidated balance sheet.
[3] | We recognize our equity securities at cost minus impairments, plus or minus changes resulting from observable price changes in orderly transactions for the identical or similar investment of the same issuer on our consolidated balance sheet.

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