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Investments (Unrealized Gains on Investments) (Details) - USD ($) $ in Thousands | 12 Months Ended |
--- | --- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018 | Dec. 31, 2017
Net unrealized gains |  | $ 386,393 | $ 215,688 | $ 2,391 |
Deferred income tax expense |  | (81,142) | (45,294) | (502) |
Net unrealized gains, net of deferred income tax |  | 305,251 | 170,394 | 1,889 |
Total stockholders’ equity |  | 2,738,889 | 2,194,936 | 1,791,802 |
Increase (decrease) in net unrealized gains in OCI, net of deferred income tax |  | 134,857 | 168,505 | (65,881) |
Accounting Standards Update 2016-01 [Member] |  |  |  |  |
Total stockholders’ equity | [1] | 0 | 0 | (30,726) |
Available-for-sale Securities [Member] |  |  |  |  |
Net unrealized gains |  | 386,380 | 215,634 | 2,302 |
Total HTM Securities [Member] |  |  |  |  |
Net unrealized gains |  | 7 | 31 | 89 |
Short-term Investments [Member] |  |  |  |  |
Net unrealized gains |  | 6 | 23 | 0 |
Investments [Member] | Accounting Standards Update 2018-02 [Member] |  |  |  |  |
Total stockholders’ equity | [1] | 0 | 0 | 17,920 |
Retained Earnings [Member] |  |  |  |  |
Total stockholders’ equity |  | $ 2,271,537 | 2,080,529 | 1,858,414 | $ 1,698,613
Retained Earnings [Member] | Accounting Standards Update 2016-01 [Member] |  |  |  |  |
Total stockholders’ equity |  |  | 0 | 0 | 30,726
Equity Impact on Cumulative Effect Adjustment |  |  |  |  | 30,700
Retained Earnings [Member] | Accounting Standards Update 2018-02 [Member] |  |  |  |  |
Total stockholders’ equity |  |  | 0 | $ 0 | $ (5,707)
Retained Earnings [Member] | Investments [Member] | Accounting Standards Update 2018-02 [Member] |  |  |  |  |
Total stockholders’ equity |  |  | $ 17,900 |  |
[1] | Upon adoption of ASU 2016-01, we recognized a $30.7 million cumulative-effect adjustment to the opening balance of AOCI, which represents the after-tax net unrealized gain on our equity portfolio as of December 31, 2018. Additionally, upon adoption of ASU 2018-02, we recognized a one-time reclassification from AOCI to retained earnings for $17.9 million representing the stranded tax assets related to our investment portfolio that were created in AOCI from the enactment of the Tax Cuts and Jobs Act of 2017.

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