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Qualified Employee Benefit Plans - Amounts Included in Accumulated Other Comprehensive Income (Loss) (Details) - Retirement Plan [Member] - USD ($)
$ in Thousands
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Unrecognized net loss from experience different from that assumed and effects of changes and assumptions $ (49,473) $ (46,430)
Prior service cost (779) (803)
Pension and other postretirement benefit plans, accumulated other comprehensive loss, before tax (50,252) (47,233)
Income tax benefit 408 457
Accumulated other comprehensive loss $ (49,844) $ (46,776)