Benefit Plans - Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension Benefits
Amounts arising during the year:
Net (gain) loss on pension assets	$ (16,278)	$ 12,209	$ (8,619)
Net loss (gain) on pension obligations	21,512	(6,619)	9,757
Reclassification adjustments recognized as components of net periodic benefit cost during the year:
Net (gain) loss	(6,995)	(7,315)	(8,625)
Amount recognized in other comprehensive income	(1,761)	(1,725)	(7,487)
Other Benefits
Amounts arising during the year:
Net loss (gain) on pension obligations	540	(2,755)	(34)
Reclassification adjustments recognized as components of net periodic benefit cost during the year:
Net (gain) loss	385
Prior service credit	429	429	429
Amount recognized in other comprehensive income	$ 1,354	$ (2,326)	$ 395