Retirement Plans and Postretirement Medical Benefits (Fair Value Measurements of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	$ 1,102,556	$ 747,177
Cash	1,009,021	764,522	$ 640,190	$ 1,043,439
Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	260,959	232,829
Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	841,597	514,348
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	1,733,997	1,622,945
Securities lending payable	(152,179)	(174,651)
Cash	5,186	18,164
Other	(29,097)	(2,376)
Fair value of plan assets	1,557,907	1,464,082
United States | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	457,029	400,926
United States | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	1,147,254	1,083,294
United States | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	129,714	138,725
United States | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	18,160	9,213
United States | Money market funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	8,810	2,604
United States | Money market funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	9,350	6,609
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	302,858	324,945
United States | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	152,815	184,254
United States | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	150,043	140,691
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	377,078	358,776
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	377,078	358,776
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	315,877	235,194
United States | Government and related securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	295,404	214,068
United States | Government and related securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	20,473	21,126
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	418,908	367,369
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	418,908	367,369
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	19,223	15,308
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	19,223	14,072
United States | Mortgage-backed securities /asset-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	1,236
United States | Private equity
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	38,362	49,637
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	38,362	49,637
United States | Real estate
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	91,352	87,852
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	91,352	87,852
United States | Securities lending collateral
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	152,179	174,651
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	152,179	174,651
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	629,880	541,463
Cash	2,203	4,262
Other	627	1,565
Fair value of plan assets	632,710	547,290
Foreign | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	32,295
Foreign | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	544,255	437,906
Foreign | Level 3
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	85,625	71,262
Foreign | Money market funds
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	13,375	6,811
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	13,375	6,811
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	226,032	214,238
Foreign | Equity securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	0	32,295
Foreign | Equity securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	226,032	181,943
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	213,844	69,022
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	213,844	69,022
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	66,115	29,363
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	66,115	29,363
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	24,889	150,767
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	24,889	150,767
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	41,601	34,483
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	41,601	34,483
Foreign | Diversified growth funds
Defined Benefit Plan Disclosure [Line Items]
Total plan assets at fair value	44,024	36,779
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	$ 44,024	$ 36,779