Benefit Plans - Change in Net Actuarial Loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Benefit plans
Percentage of greater of projected benefit obligation or fair value of plan assets for amortization of actuarial gains or losses	5.00%
Amortization period for actuarial gains and losses that exceed threshold	5 years
Pension Benefits
Change in net actuarial loss and amortization
Net actuarial loss at beginning of year	$ 51,034	$ 45,579
Amortization cost	(8,625)	(7,629)	$ (9,928)
Liability loss (gain)	9,757	9,653	(8,004)
Asset loss (gain)	(8,619)	3,350	3,700
Change due to the Reorganization Transactions		81
Net actuarial loss at end of year	43,547	51,034	45,579
Other Benefits
Change in net actuarial loss and amortization
Net actuarial loss at beginning of year	282	619
Amortization cost			(32)
Liability loss (gain)	(34)	(337)	(1,018)
Net actuarial loss at end of year	$ 248	$ 282	$ 619