Benefit Plans - Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Amounts arising during the year:
Net (gain) loss on pension assets	$ (4,839)	$ (16,278)	$ 12,209
Net loss (gain) on pension obligations	14,935	21,512	(6,619)
Reclassification adjustments recognized as components of net periodic benefit cost during the year:
Net (gain) loss	(5,806)	(6,995)	(7,315)
Amount recognized in other comprehensive income	4,290	(1,761)	(1,725)
Other Benefits
Amounts arising during the year:
Net loss (gain) on pension obligations	303	540	(2,755)
Reclassification adjustments recognized as components of net periodic benefit cost during the year:
Net (gain) loss	211	385
Prior service credit	51	429	429
Amount recognized in other comprehensive income	$ 565	$ 1,354	$ (2,326)