Retirement Plans and Other Benefits - Amounts Recognized on the Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	$ 166,206	$ 185,997
Pension Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	0	0
Current liability	(19,795)	(10,031)
Noncurrent liability	(509,310)	(480,998)
Net amount recognized	(529,105)	(491,029)
Other Benefits
Amounts recognized on the Consolidated Balance Sheets
Noncurrent asset	166,206	185,997
Current liability	0	0
Noncurrent liability	0	0
Net amount recognized	$ 166,206	$ 185,997