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Investments, Summary of Investments (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2019 | Dec. 31, 2018
Summary of Investments [Abstract] |  |  |  |
Cost |  | $ 1,212 | [1] | $ 1,212
Gross unrealized gains |  | 0 |  | 137
Gross unrealized losses |  | (244) |  | 0
Estimated fair value |  | 968 |  | 1,349
Cost |  | 1,920,577 |  | 1,874,529
Gross unrealized gains |  | 2,045 |  | 556
Gross unrealized losses |  | (1,600) |  | (4,068)
Estimated fair value |  | 1,921,022 |  | 1,871,017
Available-for-sale Securities [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 1,919,365 |  | 1,873,317
Gross unrealized gains |  | 2,045 |  | 419
Gross unrealized losses |  | (1,356) |  | (4,068)
Estimated fair value |  | 1,920,054 |  | 1,869,668
Available-for-sale Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 1,388,864 |  | 1,433,930
Gross unrealized gains |  | 518 |  | 31
Gross unrealized losses |  | (761) |  | (2,362)
Estimated fair value |  | 1,388,621 |  | 1,431,599
Available-for-sale Securities [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 530,501 |  | 439,387
Gross unrealized gains |  | 1,527 |  | 388
Gross unrealized losses |  | (595) |  | (1,706)
Estimated fair value |  | 531,433 |  | 438,069
Corporate Debt Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 869,483 | [2] | 956,879
Gross unrealized gains |  | 326 |  | 13
Gross unrealized losses |  | (531) |  | (1,858)
Estimated fair value |  | 869,278 |  | 955,034
Corporate Debt Securities [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 360,976 |  | 299,018
Gross unrealized gains |  | 1,236 |  | 194
Gross unrealized losses |  | (336) |  | (1,286)
Estimated fair value |  | 361,876 |  | 297,926
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 124,744 |  | 168,839
Gross unrealized gains |  | 71 |  | 3
Gross unrealized losses |  | (15) |  | (104)
Estimated fair value |  | 124,800 |  | 168,738
Debt Securities issued by U.S. Government Agencies [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 120,341 |  | 107,789
Gross unrealized gains |  | 279 |  | 194
Gross unrealized losses |  | (112) |  | (109)
Estimated fair value |  | 120,508 |  | 107,874
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 328,340 | [2] | 244,640
Gross unrealized gains |  | 110 |  | 15
Gross unrealized losses |  | (12) |  | (77)
Estimated fair value |  | 328,438 |  | 244,578
Debt Securities issued by the U.S. Treasury [Member] | Securities with Maturity of More than One Year [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost |  | 33,569 | [1] | 15,600
Gross unrealized gains |  | 12 |  | 0
Gross unrealized losses |  | (22) |  | (24)
Estimated fair value |  | 33,559 |  | 15,576
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],[2] | 63,366 |  | 63,572
Gross unrealized gains |  | 10 |  | 0
Gross unrealized losses |  | (203) |  | (323)
Estimated fair value |  | 63,173 |  | 63,249
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] | 15,615 |  | 16,980
Gross unrealized gains |  | 0 |  | 0
Gross unrealized losses |  | (125) |  | (287)
Estimated fair value |  | 15,490 |  | $ 16,693
Other Municipal Debt Securities [Member] | Securities with Maturity of One Year or Less [Member] |  |  |  |
Summary of Investments [Abstract] |  |  |  |
Cost | [1] | 2,931 |  |
Gross unrealized gains |  | 1 |  |
Gross unrealized losses |  | 0 |  |
Estimated fair value |  | $ 2,932 |  |
[1] | Our available-for-sale securities are held at amortized cost.
[2] | Includes investments classified as cash equivalents on our condensed consolidated balance sheet.

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