Retirement Plans and Other Benefits - Changes in Fair Value (Details) - Pension Benefits - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Changes in fair value for assets that are measured at fair value on a recurring basis
Balance at the beginning of the period	$ 2,615,404	$ 2,264,121
Balance at the end of the period	2,542,774	2,615,404
Short-term investments and other | Significant Unobservable Inputs (Level 3)
Changes in fair value for assets that are measured at fair value on a recurring basis
Balance at the beginning of the period	27,929	8,660
Actual return on assets still held	2,789	927
Purchases	13,187	19,984
Sales	(1,808)	(1,642)
Transfers in and/or out of Level 3	0	0
Balance at the end of the period	$ 42,097	$ 27,929