Retirement Plans and Postretirement Medical Benefits (Level 3 Gains and Losses) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
United States
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	$ 138,725	$ 147,738
Realized gains	10,231	11,488
Unrealized gains (losses)	112	(2,687)
Net purchases, sales and settlements	(19,354)	(17,814)
Balance at	129,714	138,725
United States | Mortgage-backed securities /asset-backed securities
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	1,236	1,592
Realized gains	25	8
Unrealized gains (losses)	49	38
Net purchases, sales and settlements	(1,310)	(402)
Balance at	0	1,236
United States | Private equity
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	49,637	63,577
Realized gains	9,226	10,200
Unrealized gains (losses)	(2,334)	(7,540)
Net purchases, sales and settlements	(18,167)	(16,600)
Balance at	38,362	49,637
United States | Real estate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	87,852	82,569
Realized gains	980	1,280
Unrealized gains (losses)	2,397	4,815
Net purchases, sales and settlements	123	(812)
Balance at	91,352	87,852
Foreign
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	71,262	59,690
Unrealized gains (losses)	5,710	3,020
Net purchases, sales and settlements	1,481	20,464
Foreign currency gain	7,172	(11,912)
Balance at	85,625	71,262
Foreign | Real estate
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	34,483	39,177
Unrealized gains (losses)	2,159	459
Net purchases, sales and settlements	1,481	1,436
Foreign currency gain	3,478	(6,589)
Balance at	41,601	34,483
Foreign | Diversified growth funds
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward]
Balance at	36,779	20,513
Unrealized gains (losses)	3,551	2,561
Net purchases, sales and settlements	0	19,028
Foreign currency gain	3,694	(5,323)
Balance at	$ 44,024	$ 36,779