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Other Comprehensive Income (Loss) - Summary of Unrealized Gains and Losses on Postretirement Benefits (Details) - USD ($)
$ in Thousands
3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Roll Forward]        
Balance at beginning of period     $ 223,726 $ 212,508
Balance at end of period $ 234,155 $ 216,584 234,155 216,584
Accumulated Defined Benefit Plans Adjustment Attributable to Parent        
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Roll Forward]        
Balance at beginning of period 28 24 28 24
Amortization of unrecognized transition obligation 0 0 0 0
Change in unamortized net actuarial gain (loss) 0 0 0 0
Related deferred taxes 0 0 0 0
Balance at end of period $ 28 $ 24 $ 28 $ 24