Pension and Other Postretirement Benefits - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension Benefits [Member]
Components of net periodic benefit cost:
Service cost	$ 4,978	$ 4,981	$ 5,505
Interest cost	7,671	8,909	9,116
Expected return on plan assets	(10,644)	(12,013)	(12,518)
Amortization of prior service credit	(41)	(42)	(44)
Curtailment gain	0	(227)	(128)
Settlement loss (gain)	(8)	7,630	128
Net loss recognition	2,597	2,670	5,082
Net periodic benefit cost	4,553	11,908	7,141
Other Benefits [Member]
Components of net periodic benefit cost:
Service cost	49	46	52
Interest cost	1,139	1,259	1,301
Expected return on plan assets	0	0	0
Amortization of prior service credit	0	(42)	(87)
Curtailment gain	0	0	0
Settlement loss (gain)	0	0	0
Net loss recognition	0	86	328
Net periodic benefit cost	$ 1,188	$ 1,349	$ 1,594