Retirement Plans and Other Benefits - Changes in Fair Value (Details) (Pension Benefits, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Changes in fair value for assets that are measured at fair value on a recurring basis
Balance at the beginning of the period			$ 2,079,181
Balance at the end of the period	2,615,404	2,264,121	2,079,181
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	8,660	2,419
Actual return on assets still held	927	(498)
Purchases	19,984	7,377
Sales	(1,642)	(638)
Transfers in and/or out of Level 3	0	0
Balance at the end of the period	$ 27,929	$ 8,660