Retirement Plans and Other Benefits - Amounts Recognized on the Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	$ 46,906	$ 268,978
Pension Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	0	0
Current liability	(13,980)	(9,859)
Noncurrent liability	(443,170)	(327,300)
Net amount recognized	(457,150)	(337,159)
Other Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	46,906	268,978
Current liability	0	0
Noncurrent liability	0	0
Net amount recognized	$ 46,906	$ 268,978