Retirement Plans and Other Benefits - Amounts Recognized on the Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	$ 268,978	$ 166,206
Pension Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	0	0
Current liability	(9,859)	(19,795)
Noncurrent liability	(327,300)	(509,310)
Net amount recognized	(337,159)	(529,105)
Other Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	268,978	166,206
Current liability	0	0
Noncurrent liability	0	0
Net amount recognized	$ 268,978	$ 166,206