Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension Benefits
Components of net periodic benefit cost:
Service cost	$ 4,705	$ 4,978	$ 4,981
Interest cost	11,690	7,671	8,909
Expected return on plan assets	(16,391)	(10,644)	(12,013)
Amortization of prior service credit	(42)	(41)	(42)
Curtailment gain	0	0	(227)
Settlement loss (gain)	1,342	(8)	7,630
Net loss (gain) recognition	2,810	2,597	2,670
Net periodic benefit cost	4,114	4,553	11,908
Other Benefits
Components of net periodic benefit cost:
Service cost	47	49	46
Interest cost	945	1,139	1,259
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	0	(42)
Curtailment gain	0	0	0
Settlement loss (gain)	0	0	0
Net loss (gain) recognition	(12)	0	86
Net periodic benefit cost	$ 980	$ 1,188	$ 1,349