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Comprehensive Income (Reclassification out of AOCI) (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2016 | Sep. 30, 2016 | Jun. 30, 2016 | Mar. 31, 2016 | Dec. 31, 2015 | Sep. 30, 2015 | Jun. 30, 2015 | Mar. 31, 2015 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2014
Realized investment (losses) gains |  |  |  |  |  |  |  |  | $ (4,937) | $ 13,171 | $ 26,599
Net investment income | $ 35,428 | $ 33,375 | $ 31,182 | $ 30,769 | $ 30,108 | $ 32,061 | $ 32,230 | $ 26,917 | 130,754 | 121,316 | 138,708
Income from continuing operations, before federal income tax |  |  |  |  |  |  |  |  | 219,955 | 232,692 | 197,131
Other Comprehensive (Income) Loss, Pension and Other Postretirement Benefit Plans, Adjustment, before Tax |  |  |  |  |  |  |  |  | 5,617 | (9,515) | 52,234
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (61,460) | (66,831) | (55,304)
Net income | $ 39,360 | $ 38,502 | $ 43,601 | $ 37,032 | $ 45,389 | $ 46,996 | $ 33,768 | $ 39,708 | 158,495 | 165,861 | 141,827
Losses and loss expenses incurred |  |  |  |  |  |  |  |  | 1,234,797 | 1,148,541 | 1,157,501
Policy acquisition costs |  |  |  |  |  |  |  |  | 763,758 | 689,820 | 624,470
Total defined benefit pension and post-retirement plans |  |  |  |  |  |  |  |  | (3,652) | 6,185 | $ (33,953)
Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Net income |  |  |  |  |  |  |  |  | 7,310 | (4,655) |
Reclassification out of Accumulated Other Comprehensive Income [Member] | Retirement Income Plan [Member] |  |  |  |  |  |  |  |  |  |  |
Income from continuing operations, before federal income tax |  |  |  |  |  |  |  |  | 6,462 | 7,077 |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (2,262) | (2,477) |
Net income |  |  |  |  |  |  |  |  | 4,200 | 4,600 |
Losses and loss expenses incurred |  |  |  |  |  |  |  |  | 1,486 | 1,538 |
Policy acquisition costs |  |  |  |  |  |  |  |  | 4,976 | 5,539 |
Accumulated other-than-temporary impairments including portion attributable to noncontrolling interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Realized investment (losses) gains |  |  |  |  |  |  |  |  | 213 | 357 |
Income from continuing operations, before federal income tax |  |  |  |  |  |  |  |  | 213 | 357 |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (75) | (125) |
Net income |  |  |  |  |  |  |  |  | 138 | 232 |
Held-to-maturity Securities [Member] | Accumulated net investment gain (loss) including portion attributable to noncontrolling interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Realized investment (losses) gains |  |  |  |  |  |  |  |  | 169 | 308 |
Net investment income |  |  |  |  |  |  |  |  | (310) | (888) |
Income from continuing operations, before federal income tax |  |  |  |  |  |  |  |  | (141) | (580) |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | 49 | 203 |
Net income |  |  |  |  |  |  |  |  | (92) | (377) |
Available-for-sale Securities [Member] | Accumulated net investment gain (loss) including portion attributable to noncontrolling interest [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member] |  |  |  |  |  |  |  |  |  |  |
Realized investment (losses) gains |  |  |  |  |  |  |  |  | 4,713 | (14,016) |
Income from continuing operations, before federal income tax |  |  |  |  |  |  |  |  | 4,713 | (14,016) |
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  | (1,649) | 4,906 |
Net income |  |  |  |  |  |  |  |  | $ 3,064 | $ (9,110) |

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