Employee Benefit Plans - Net Periodic Benefit Pension Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Components of net periodic benefit cost
Interest cost	$ 1,712	$ 1,635	$ 2,334
Expected return on plan assets	(3,802)	(4,057)	(4,333)
Recognized net actuarial loss	591	571	367
Amortization of prior service cost	0	0	0
Recognized settlement loss	2,480	1,338	2,254
Net periodic cost (income)	981	(513)	622
Other changes in plan assets and benefit obligations recognized as a charge to other comprehensive income (loss)
Net loss for the period	(83)	(3,911)	(9,191)
Amortization of net loss	3,071	1,909	2,621
Total unrealized gain (loss)	2,988	(2,002)	(6,570)
Total recognized in net periodic pension cost and other comprehensive income (loss)	$ 2,007	$ (1,489)	$ (7,192)
Weighted-average assumptions used to determine the net periodic cost
Discount rate	3.86%	3.99%	3.60%
Expected return on plan assets	6.25%	6.50%	6.50%