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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2018 | Sep. 30, 2018 | Jun. 30, 2018 | Mar. 31, 2018 | Dec. 31, 2017 | Sep. 30, 2017 | Jun. 30, 2017 | Mar. 31, 2017 | Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016
Deferred policy acquisition costs | $ 252,612 |  |  |  |  | $ 235,055 |  |  |  |  | $ 252,612 | $ 235,055 | $ 222,564
Reserve for loss and loss expenses | 3,893,868 |  |  |  |  | 3,771,240 |  |  |  |  | 3,893,868 | 3,771,240 | 3,691,719
Unearned premiums | 1,431,932 |  |  |  |  | 1,349,644 |  |  |  |  | 1,431,932 | 1,349,644 | 1,262,819
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 2,436,229 | 2,291,027 | 2,149,572
Net investment income |  |  |  |  |  |  |  |  |  |  | 140,413 | 168,241 | 125,817
Loss and loss expense incurred |  |  |  |  |  |  |  |  |  |  | 1,498,134 | 1,345,074 | 1,234,797
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  |  |  | 495,042 | 469,236 | 450,328
Other operating expenses |  |  |  |  |  |  |  |  |  |  | 321,880 | 322,381 | 312,514
Net premiums written |  |  |  |  |  |  |  |  |  |  | 2,514,286 | 2,370,641 | 2,237,288
Other insurance expenses |  |  |  |  |  |  |  |  |  |  | 331,318 | 333,097 | 321,395
Other income | (1,542) | [1] | $ (2,538) | $ (3,179) | $ (2,179) | (2,190) | [1] | $ (1,994) | $ (3,291) | $ (3,241) | (9,438) | (10,716) | (8,881)
Standard Commercial Lines [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 206,391 |  |  |  |  | 193,408 |  |  |  |  | 206,391 | 193,408 | 181,193
Reserve for loss and loss expenses | 3,283,531 |  |  |  |  | 3,165,217 |  |  |  |  | 3,283,531 | 3,165,217 | 3,098,554
Unearned premiums | 1,020,054 |  |  |  |  | 956,173 |  |  |  |  | 1,020,054 | 956,173 | 884,976
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 1,912,222 | 1,788,499 | 1,665,483
Net investment income |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Loss and loss expense incurred |  |  |  |  |  |  |  |  |  |  | 1,141,038 | 1,008,150 | 913,506
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  |  |  | 412,420 | 387,552 | 367,813
Other operating expenses |  |  |  |  |  |  |  |  |  |  | 249,660 | 243,283 | 237,729
Net premiums written |  |  |  |  |  |  |  |  |  |  | 1,975,683 | 1,858,735 | 1,745,782
Standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 18,070 |  |  |  |  | 16,952 |  |  |  |  | 18,070 | 16,952 | 16,664
Reserve for loss and loss expenses | 223,223 |  |  |  |  | 263,166 |  |  |  |  | 223,223 | 263,166 | 286,081
Unearned premiums | 304,085 |  |  |  |  | 295,435 |  |  |  |  | 304,085 | 295,435 | 282,111
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 304,441 | 289,701 | 280,607
Net investment income |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Loss and loss expense incurred |  |  |  |  |  |  |  |  |  |  | 206,752 | 189,294 | 177,749
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  |  |  | 33,617 | 32,542 | 34,105
Other operating expenses |  |  |  |  |  |  |  |  |  |  | 51,308 | 56,761 | 56,334
Net premiums written |  |  |  |  |  |  |  |  |  |  | 309,277 | 296,775 | 281,822
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 28,151 |  |  |  |  | 24,695 |  |  |  |  | 28,151 | 24,695 | 24,707
Reserve for loss and loss expenses | 387,114 |  |  |  |  | 342,857 |  |  |  |  | 387,114 | 342,857 | 307,084
Unearned premiums | 107,793 |  |  |  |  | 98,036 |  |  |  |  | 107,793 | 98,036 | 95,732
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 219,566 | 212,827 | 203,482
Net investment income |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Loss and loss expense incurred |  |  |  |  |  |  |  |  |  |  | 150,344 | 147,630 | 143,542
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  |  |  | 49,005 | 49,142 | 48,410
Other operating expenses |  |  |  |  |  |  |  |  |  |  | 20,912 | 22,337 | 18,451
Net premiums written |  |  |  |  |  |  |  |  |  |  | 229,326 | 215,131 | 209,684
Investments Segment [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 0 |  |  |  |  | 0 |  |  |  |  | 0 | 0 | 0
Reserve for loss and loss expenses | 0 |  |  |  |  | 0 |  |  |  |  | 0 | 0 | 0
Unearned premiums | $ 0 |  |  |  |  | $ 0 |  |  |  |  | 0 | 0 | 0
Net premiums earned |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net investment income |  |  |  |  |  |  |  |  |  |  | 140,413 | 168,241 | 125,817
Loss and loss expense incurred |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Other operating expenses |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Net premiums written |  |  |  |  |  |  |  |  |  |  | 0 | 0 | 0
Miscellaneous Income [Member] |  |  |  |  |  |  |  |  |  |  |  |  |
Other income |  |  |  |  |  |  |  |  |  |  | $ (9,438) | $ (10,716) | $ (8,881)
[1] | Results for the fourth quarter of 2017 include the impact of the $20.2 million write off of deferred tax assets required with the implementation of Tax Reform. See Note 13. "Federal Income Taxes" above for additional information.

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