Accumulated Other Comprehensive Earnings (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2016	Dec. 26, 2015	Dec. 27, 2014
Accumulated Other Comprehensive Earnings (Losses), Net of Tax [Roll Forward]
Balances at beginning of period	$ 26,268	$ 109,134	$ 86,999
Unrealized gain on AFS securities	17,615	(43,838)	58,968
Net realized gain on AFS securities reclassified to investment income	(19,792)	(42,829)	(35,842)
Amortization of actuarial loss reclassified to operating and administrative expenses	(664)	3,801	(991)
Net other comprehensive earnings (losses)	(2,841)	(82,866)	22,135
Balances at end of period	23,427	26,268	109,134
AFS Securities
Accumulated Other Comprehensive Earnings (Losses), Net of Tax [Roll Forward]
Balances at beginning of period	31,295	117,962	94,836
Unrealized gain on AFS securities	17,615	(43,838)	58,968
Net realized gain on AFS securities reclassified to investment income	(19,792)	(42,829)	(35,842)
Net other comprehensive earnings (losses)	(2,177)	(86,667)	23,126
Balances at end of period	29,118	31,295	117,962
Postretirement Benefit
Accumulated Other Comprehensive Earnings (Losses), Net of Tax [Roll Forward]
Balances at beginning of period	(5,027)	(8,828)	(7,837)
Amortization of actuarial loss reclassified to operating and administrative expenses	(664)	3,801	(991)
Net other comprehensive earnings (losses)	(664)	3,801	(991)
Balances at end of period	$ (5,691)	$ (5,027)	$ (8,828)

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

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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.

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Amount after tax and reclassification adjustments of other comprehensive income (loss).

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Amount after tax of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.

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Amount after tax of reclassification adjustment from accumulated other comprehensive income (loss) for actuarial gain (loss) related to pension and other postretirement benefit plans.

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Amount after tax, before reclassification adjustments, of unrealized holding gain (loss) on available-for-sale securities.

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