Retirement Plans and Postretirement Medical Benefits (Fair Value Measurements of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	$ 919,371	$ 1,102,556
Cash and cash equivalents	867,262	1,009,021	$ 764,522
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	321,116	260,959
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	598,255	841,597
Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	1,491,887	1,733,997
Securities lending payable	(117,603)	(152,179)
Cash and cash equivalents	11,341	5,186
Other	(58,591)	(29,097)
Fair value of plan assets	1,327,034	1,557,907
United States | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	372,873	457,029
United States | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	989,387	1,147,254
United States | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	129,627	129,714
United States | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	9,257	18,160
United States | Money market funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	3,498	8,810
United States | Money market funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	5,759	9,350
United States | Money market funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	220,704	302,858
United States | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	110,840	152,815
United States | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	109,864	150,043
United States | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Commingled fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	281,258	377,078
United States | Commingled fixed income securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Commingled fixed income securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	281,258	377,078
United States | Commingled fixed income securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Government and related securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	274,679	315,877
United States | Government and related securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	258,535	295,404
United States | Government and related securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	16,144	20,473
United States | Government and related securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	435,285	418,908
United States | Corporate debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Corporate debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	435,285	418,908
United States | Corporate debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Mortgage-backed securities /asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	23,474	19,223
United States | Mortgage-backed securities /asset-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Mortgage-backed securities /asset-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	23,474	19,223
United States | Mortgage-backed securities /asset-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	32,750	38,362
United States | Private equity | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Private equity | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Private equity | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	32,750	38,362
United States | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	96,877	91,352
United States | Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	96,877	91,352
United States | Securities lending collateral
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	117,603	152,179
United States | Securities lending collateral | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
United States | Securities lending collateral | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	117,603	152,179
United States | Securities lending collateral | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	557,948	629,880
Cash and cash equivalents	3,903	2,203
Other	666	627
Fair value of plan assets	562,517	632,710
Foreign | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	475,039	544,255
Foreign | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	82,909	85,625
Foreign | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	11,172	13,375
Foreign | Money market funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Money market funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	11,172	13,375
Foreign | Money market funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	194,914	226,032
Foreign | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	194,914	226,032
Foreign | Equity securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Commingled fixed income securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	198,902	213,844
Foreign | Commingled fixed income securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Commingled fixed income securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	198,902	213,844
Foreign | Commingled fixed income securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Government and related securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	40,055	66,115
Foreign | Government and related securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Government and related securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	40,055	66,115
Foreign | Government and related securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Corporate debt securities
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	29,996	24,889
Foreign | Corporate debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Corporate debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	29,996	24,889
Foreign | Corporate debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	42,143	41,601
Foreign | Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	42,143	41,601
Foreign | Diversified growth funds
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	40,766	44,024
Foreign | Diversified growth funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Diversified growth funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	0
Foreign | Diversified growth funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	$ 40,766	$ 44,024