Benefit Plans - Amounts Recognized in Other Comprehensive Income (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits
Amounts arising during the year:
Net loss (gain) on pension assets	$ (8,619)	$ 3,350	$ 3,700
Net loss (gain) on pension obligations	9,757	9,653	(8,004)
Change due to the Reorganization Transactions		81
Reclassification adjustments recognized as components of net periodic benefit cost during the year:
Net loss	(8,625)	(7,629)	(9,928)
Amount recognized in other comprehensive income	(7,487)	5,455	(14,232)
Other Benefits
Amounts arising during the year:
Net loss (gain) on pension obligations	(34)	(337)	(1,018)
Reclassification adjustments recognized as components of net periodic benefit cost during the year:
Net loss			(32)
Prior service credit	429	429	429
Amount recognized in other comprehensive income	$ 395	$ 92	$ (621)