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Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract] |  |  |  |  |  |  |  |  |  |  |
Net premiums earned | $ 237,425 | $ 237,083 | $ 231,136 | $ 229,103 | $ 235,053 | $ 237,376 | $ 231,456 | $ 221,341 | $ 934,747 | $ 925,226 | $ 896,222
Investment income, net of expenses | 31,276 | 30,402 | 29,716 | 29,477 | 28,094 | 27,515 | 27,248 | 27,809 | 120,871 | 110,666 | 103,741
Realized gains (losses) | 460 | (47) | (42) | (122) | (52) | 5,092 | 836 | 3,056 | 249 | 8,932 | 28,361
Other revenue | 2,341 | 2,922 | 2,502 | 2,422 | 3,425 | 3,867 | 3,994 | 6,373 | 10,187 | 17,659 | 12,964
Loss incurred, net | (30,996) | 29,747 | 27,339 | 27,619 | 47,658 | 60,897 | 46,590 | 85,012 | 53,709 | 240,157 | 343,547
Underwriting and other expenses, net | 57,042 | 56,146 | 55,292 | 59,304 | 56,824 | 53,981 | 49,837 | 56,439 | 227,784 | 217,081 |
Loss on debt extinguishment | 0 | 0 | 65 | 0 | 0 | 75,223 | 1,868 | 13,440 | 65 | 90,531 | 507
Provision for income tax | 218,142 | 64,440 | 61,994 | 84,159 | 54,551 | 27,131 | 56,018 | 34,497 | 428,735 | 172,197 | (684,313)
Net income | $ 27,314 | $ 120,027 | $ 118,622 | $ 89,798 | $ 107,487 | $ 56,618 | $ 109,221 | $ 69,191 | $ 355,761 | $ 342,517 | $ 1,172,000
Earnings per Share [Abstract] |  |  |  |  |  |  |  |  |  |  |
Basic (in dollars per share) | $ 0.07 | $ 0.32 | $ 0.32 | $ 0.26 | $ 0.31 | $ 0.16 | $ 0.32 | $ 0.20 | $ 0.98 | $ 1.00 | $ 3.45
Diluted (in dollars per share) | $ 0.07 | $ 0.32 | $ 0.31 | $ 0.24 | $ 0.28 | $ 0.14 | $ 0.26 | $ 0.17 | $ 0.95 | $ 0.86 | $ 2.60

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