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Comprehensive Income (Components of Comprehensive Income-Gross and Net of Tax) (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
Income before federal income tax | $ 52,135 |  | $ 67,130 | $ 72,525 | $ 19,931 | $ 68,150 |  | $ 67,315 | $ 58,929 | $ 67,574 | $ 211,721 | $ 261,968 | $ 219,955
Income Tax Expense (Benefit) |  |  |  |  |  |  |  |  |  |  | 32,782 | 93,142 | 61,460
Net income | $ 45,760 | [1] | $ 55,435 | $ 58,819 | $ 18,925 | $ 30,242 | [1] | $ 46,718 | $ 41,426 | $ 50,440 | 178,939 | 168,826 | 158,495
Unrealized holding gains (losses) during the period, Gross |  |  |  |  |  |  |  |  |  |  | (123,145) | 66,894 | (9,195)
Unrealized holding gains (losses) during the period, Tax |  |  |  |  |  |  |  |  |  |  | (25,861) | 23,879 | (3,218)
Unrealized holding gains (losses) arising during period, Net |  |  |  |  |  |  |  |  |  |  | (97,284) | 43,015 | (5,977)
Non-credit portion of other-than-temporary impairment losses recognized in other comprehensive income, Gross |  |  |  |  |  |  |  |  |  |  |  | 36 | (10)
Non-Credit Portion of Other-Than-Temporary Impairment Losses Recognized In Other Comprehensive Income, Tax |  |  |  |  |  |  |  |  |  |  |  | 13 | (4)
Non-credit portion of other-than-temporary impairments recognized in other comprehensive income, Net |  |  |  |  |  |  |  |  |  |  |  | 23 | (6)
Amount reclassified into net income: HTM securities, Gross |  |  |  |  |  |  |  |  |  |  | 110 | (179) | (141)
Amounts reclassified into net income: HTM securities, Tax |  |  |  |  |  |  |  |  |  |  | 23 | (63) | (49)
Amount reclassified into net income: HTM securities, Net |  |  |  |  |  |  |  |  |  |  | 87 | (116) | (92)
Non-credit OTTI, Gross |  |  |  |  |  |  |  |  |  |  |  | 104 | 213
Non-credit OTTI, Tax |  |  |  |  |  |  |  |  |  |  |  | 36 | 75
Amounts reclassified into net income: Non-credit other-than-temporary impairment |  |  |  |  |  |  |  |  |  |  | 0 | 68 | 138
Realized (gains) losses on AFS securities, Gross |  |  |  |  |  |  |  |  |  |  | 39,641 | (6,979) | 4,713
Realized (gains) losses on AFS securities, Tax |  |  |  |  |  |  |  |  |  |  | 8,325 | (2,442) | 1,649
Realized (gains) losses on available for sale securities |  |  |  |  |  |  |  |  |  |  | 31,316 | (4,537) | 3,064
Net unrealized gains (losses), Gross |  |  |  |  |  |  |  |  |  |  | (83,394) | 59,876 | (4,420)
Net unrealized gains (losses), Tax |  |  |  |  |  |  |  |  |  |  | (17,513) | 21,423 | (1,547)
Total unrealized (losses) gains on investment securities |  |  |  |  |  |  |  |  |  |  | (65,881) | 38,453 | (2,873)
Net actuarial (loss) gain, Gross |  |  |  |  |  |  |  |  |  |  | (11,273) | (4,684) | (12,079)
Net actuarial (loss) gain, Tax |  |  |  |  |  |  |  |  |  |  | (2,367) | (984) | (4,227)
Net actuarial (loss) gain, Net |  |  |  |  |  |  |  |  |  |  | (8,906) | (3,700) | (7,852)
Amount reclassified into net i ncome: Net actuarial loss, Gross |  |  |  |  |  |  |  |  |  |  | 2,127 | 2,102 | 6,462
Amount reclassified into Net Income: Net actuarial loss, Tax |  |  |  |  |  |  |  |  |  |  | 447 | 735 | 2,262
Amount reclassified into net income: Net actuarial loss, Net |  |  |  |  |  |  |  |  |  |  | 1,680 | 1,367 | 4,200
Defined pension and other post-retirement benefit plans, Gross |  |  |  |  |  |  |  |  |  |  | (9,146) | (2,582) | (5,617)
Defined pension and other post-retirement benefit plans, Tax |  |  |  |  |  |  |  |  |  |  | (1,920) | (249) | (1,965)
Total defined benefit pension and post-retirement plans |  |  |  |  |  |  |  |  |  |  | (7,226) | (2,333) | (3,652)
Other Comprehensive Income (Loss), Gross |  |  |  |  |  |  |  |  |  |  | (92,540) | 57,294 | (10,037)
Other Comprehensive Income (Loss), Tax |  |  |  |  |  |  |  |  |  |  | (19,433) | 21,174 | (3,512)
Other comprehensive (loss) income |  |  |  |  |  |  |  |  |  |  | (73,107) | 36,120 | (6,525)
Comprehensive Income, Before Tax |  |  |  |  |  |  |  |  |  |  | 119,181 | 319,262 | 209,918
Comprehensive Income, Tax |  |  |  |  |  |  |  |  |  |  | 13,349 | 114,316 | 57,948
Comprehensive income |  |  |  |  |  |  |  |  |  |  | $ 105,832 | $ 204,946 | $ 151,970
[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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