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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ 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
Deferred policy acquisition costs | $ 235,055 |  |  |  | $ 222,564 |  |  |  | $ 235,055 | $ 222,564 | $ 213,159
Reserve for loss and loss expenses | 3,771,240 |  |  |  | 3,691,719 |  |  |  | 3,771,240 | 3,691,719 | 3,517,728
Unearned premiums | 1,349,644 |  |  |  | 1,262,819 |  |  |  | 1,349,644 | 1,262,819 | 1,169,710
Net premiums earned |  |  |  |  |  |  |  |  | 2,291,027 | 2,149,572 | 1,989,909
Net investment income |  |  |  |  |  |  |  |  | 168,241 | 125,817 | 134,487
Loss and loss expense incurred |  |  |  |  |  |  |  |  | 1,345,074 | 1,234,797 | 1,148,541
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  | 469,236 | 450,328 | 399,436
Other operating expenses |  |  |  |  |  |  |  |  | 322,381 | 312,514 | 292,903
Net premiums written |  |  |  |  |  |  |  |  | 2,370,641 | 2,237,288 | 2,069,904
Other insurance expenses |  |  |  |  |  |  |  |  | 333,097 | 321,395 | 300,359
Other income | (2,190) | $ (1,994) | $ (3,291) | $ (3,241) | (1,863) | $ (2,199) | $ (3,868) | $ (951) | (10,716) | (8,881) | (7,456)
Standard Commercial Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 193,408 |  |  |  | 181,193 |  |  |  | 193,408 | 181,193 | 171,476
Reserve for loss and loss expenses | 3,165,217 |  |  |  | 3,098,554 |  |  |  | 3,165,217 | 3,098,554 | 2,998,749
Unearned premiums | 956,173 |  |  |  | 884,976 |  |  |  | 956,173 | 884,976 | 803,648
Net premiums earned |  |  |  |  |  |  |  |  | 1,788,499 | 1,665,483 | 1,529,442
Net investment income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Loss and loss expense incurred |  |  |  |  |  |  |  |  | 1,008,150 | 913,506 | 819,573
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  | 387,552 | 367,813 | 323,754
Other operating expenses |  |  |  |  |  |  |  |  | 243,283 | 237,729 | 221,619
Net premiums written |  |  |  |  |  |  |  |  | 1,858,735 | 1,745,782 | 1,596,965
Standard Personal Lines [Member] |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 16,952 |  |  |  | 16,664 |  |  |  | 16,952 | 16,664 | 17,258
Reserve for loss and loss expenses | 263,166 |  |  |  | 286,081 |  |  |  | 263,166 | 286,081 | 265,054
Unearned premiums | 295,435 |  |  |  | 282,111 |  |  |  | 295,435 | 282,111 | 276,533
Net premiums earned |  |  |  |  |  |  |  |  | 289,701 | 280,607 | 288,134
Net investment income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Loss and loss expense incurred |  |  |  |  |  |  |  |  | 189,294 | 177,749 | 200,237
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  | 32,542 | 34,105 | 33,638
Other operating expenses |  |  |  |  |  |  |  |  | 56,761 | 56,334 | 52,923
Net premiums written |  |  |  |  |  |  |  |  | 296,775 | 281,822 | 283,926
Excess And Surplus Operations [Member] |  |  |  |  |  |  |  |  |  |  |
Deferred policy acquisition costs | 24,695 |  |  |  | 24,707 |  |  |  | 24,695 | 24,707 | 24,425
Reserve for loss and loss expenses | 342,857 |  |  |  | 307,084 |  |  |  | 342,857 | 307,084 | 253,925
Unearned premiums | 98,036 |  |  |  | 95,732 |  |  |  | 98,036 | 95,732 | 89,529
Net premiums earned |  |  |  |  |  |  |  |  | 212,827 | 203,482 | 172,333
Net investment income |  |  |  |  |  |  |  |  | 0 | 0 | 0
Loss and loss expense incurred |  |  |  |  |  |  |  |  | 147,630 | 143,542 | 128,731
Amortization of deferred policy acquisition costs |  |  |  |  |  |  |  |  | 49,142 | 48,410 | 42,044
Other operating expenses |  |  |  |  |  |  |  |  | 22,337 | 18,451 | 18,361
Net premiums written |  |  |  |  |  |  |  |  | 215,131 | 209,684 | 189,013
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 |  |  |  |  |  |  |  |  | 168,241 | 125,817 | 134,487
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 |  |  |  |  |  |  |  |  | $ (10,716) | $ (8,881) | $ (7,456)

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