Retirement Plans and Other Benefits - Amounts Recognized on the Consolidated Balance Sheets (Details) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Amounts recognized on the Consolidated Balance Sheets
Assets for other postretirement benefits	$ 185,997	$ 152,290
Pension Benefits
Amounts recognized on the Consolidated Balance Sheets
Assets for other postretirement benefits	0	0
Current liability	(10,031)	(9,508)
Noncurrent liability	(480,998)	(453,736)
Net amount recognized	(491,029)	(463,244)
Other Benefits
Amounts recognized on the Consolidated Balance Sheets
Assets for other postretirement benefits	185,997	152,290
Current liability	0	0
Noncurrent liability	0	0
Net amount recognized	$ 185,997	$ 152,290