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Shareholders' Equity (Details) - USD ($) $ / shares in Units, $ in Thousands |  | 3 Months Ended | 6 Months Ended |
--- | --- | --- | --- | ---
Jul. 24, 2018 | Jun. 30, 2018 | Jun. 30, 2017 | Jun. 30, 2018 | Jun. 30, 2017 | Dec. 31, 2017
Dividends declared (in dollars per share) |  | $ 0.45 | $ 0.44 | $ 0.90 |  | $ 0.88 |  |
Schedule of Accumulated Other Income (Loss) [Abstract] |  |  |  |  |  |  |  |
Balance, Beginning of Period |  |  |  | $ (36,174) |  | $ (10,967) |  |
Transition adjustment for unrealized gains and losses on equity securities |  |  |  |  |  |  |  | $ (2,709)
Net change in unrealized gain (loss) |  | $ (33,117) | $ 21,958 | (130,523) |  | 33,369 |  |
Reclassification adjustments included in earnings: |  |  |  |  |  |  |  |
Loss (gain) on available for sale securities, net |  | 762 | (380) | 1,052 |  | (2,429) |  |
Other comprehensive income (loss), before income taxes |  | (32,355) | 21,578 | (129,471) |  | 30,940 |  |
Federal and state income taxes |  | (8,241) | 8,393 | (33,049) | [1] | 12,009 | [2] |
Other comprehensive income (loss), net of income taxes |  | (24,114) | 13,185 | (96,422) |  | 18,931 |  |
Balance, End of Period |  | (135,305) | 7,964 | (135,305) |  | 7,964 |  |
Accumulated Net Unrealized Gain (Loss) on Available for Sale Securities [Member] |  |  |  |  |  |  |  |
Schedule of Accumulated Other Income (Loss) [Abstract] |  |  |  |  |  |  |  |
Balance, Beginning of Period |  |  |  | (35,385) |  | (9,087) |  |
Transition adjustment for unrealized gains and losses on equity securities |  |  |  |  |  |  |  | (2,709)
Net change in unrealized gain (loss) |  |  |  | (130,523) |  | 33,369 |  |
Reclassification adjustments included in earnings: |  |  |  |  |  |  |  |
Loss (gain) on available for sale securities, net |  |  |  | 1,052 |  | (2,429) |  |
Other comprehensive income (loss), before income taxes |  |  |  | (129,471) |  | 30,940 |  |
Federal and state income taxes |  |  |  | (33,049) | [1] | 12,009 | [2] |
Other comprehensive income (loss), net of income taxes |  |  |  | (96,422) |  | 18,931 |  |
Balance, End of Period |  | (134,516) | 9,844 | (134,516) |  | 9,844 |  |
Accumulated Unrealized Gain (Loss) on Employee Benefit Plans [Member] |  |  |  |  |  |  |  |
Schedule of Accumulated Other Income (Loss) [Abstract] |  |  |  |  |  |  |  |
Balance, Beginning of Period |  |  |  | (789) |  | (1,880) |  |
Transition adjustment for unrealized gains and losses on equity securities |  |  |  |  |  |  |  | $ 0
Net change in unrealized gain (loss) |  |  |  | 0 |  | 0 |  |
Reclassification adjustments included in earnings: |  |  |  |  |  |  |  |
Loss (gain) on available for sale securities, net |  |  |  | 0 |  | 0 |  |
Other comprehensive income (loss), before income taxes |  |  |  | 0 |  | 0 |  |
Federal and state income taxes |  |  |  | 0 | [1] | 0 | [2] |
Other comprehensive income (loss), net of income taxes |  |  |  | 0 |  | 0 |  |
Balance, End of Period |  | $ (789) | $ (1,880) | $ (789) |  | $ (1,880) |  |
Subsequent Event [Member] |  |  |  |  |  |  |  |
Dividends Payable, Date Declared | Jul. 24, 2018 |  |  |  |  |  |  |
Dividends declared (in dollars per share) | $ 0.50 |  |  |  |  |  |  |
Dividends Payable, Date to be Paid | Aug. 27, 2018 |  |  |  |  |  |  |
Dividends Payable, Date of Record | Aug. 13, 2018 |  |  |  |  |  |  |
[1] | Calculated using a 25 percent blended federal and state statutory tax rate.
[2] | Calculated using a 39 percent blended federal and state statutory tax rate.

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- Definition Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 11 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e637-108580 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14 -URI http://asc.fasb.org/extlink&oid=84228862&loc=d3e681-108580 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 220 -SubTopic 10 -Section 45 -Paragraph 14A -URI http://asc.fasb.org/extlink&oid=84228862&loc=SL7669686-108580 + Details
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