Retirement Plans and Postretirement Medical Benefits (Fair Value Measurements of Plan Assets) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Measured at Net Asset Value Per Share
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 17,132	$ 23,608
United States | Total plan assets at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,601,786	1,487,018
United States | Total plan assets at fair value | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	322,851	292,506
United States | Total plan assets at fair value | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,409,106	1,248,929
United States | Total plan assets at fair value | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69,347	71,337
United States | Total plan assets at fair value | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,801,304	1,612,772
United States | Money market funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Money market funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14,442	4,917
United States | Money market funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Money market funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14,442	4,917
United States | Equities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Equities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	323,311	265,832
United States | Equities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Equities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	323,311	265,832
United States | Commingled fixed income securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Commingled fixed income securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	264,896	275,335
United States | Commingled fixed income securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Commingled fixed income securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	264,896	275,335
United States | Government and related securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	322,851	292,506
United States | Government and related securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22,549	15,764
United States | Government and related securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Government and related securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	345,400	308,270
United States | Corporate debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Corporate debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	586,998	528,425
United States | Corporate debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Corporate debt securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	586,998	528,425
United States | Mortgage-backed /asset-backed securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Mortgage-backed /asset-backed securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45,861	51,770
United States | Mortgage-backed /asset-backed securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Mortgage-backed /asset-backed securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45,861	51,770
United States | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45,275	45,335	$ 42,143
United States | Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69,347	71,337
United States | Real estate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	69,347	71,337
United States | Diversified growth funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,750	47,621	$ 40,766
United States | Securities lending collateral | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Securities lending collateral | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151,049	106,886
United States | Securities lending collateral | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
United States | Securities lending collateral | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151,049	106,886
United States | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15,449	9,409
United States | Securities lending payable
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	151,049	106,886
United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	81,050	51,885
Foreign | Total plan assets at fair value
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	742,639	668,308
Foreign | Total plan assets at fair value | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Total plan assets at fair value | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	639,681	573,352
Foreign | Total plan assets at fair value | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	96,025	92,956
Foreign | Total plan assets at fair value | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	735,706	666,308
Foreign | Money market funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Money market funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,072	8,734
Foreign | Money market funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Money market funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10,072	8,734
Foreign | Equities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Equities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	166,683	222,554
Foreign | Equities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Equities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	166,683	222,554
Foreign | Commingled fixed income securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Commingled fixed income securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	379,656	264,131
Foreign | Commingled fixed income securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Commingled fixed income securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	379,656	264,131
Foreign | Government and related securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Government and related securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46,268	43,405
Foreign | Government and related securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Government and related securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46,268	43,405
Foreign | Corporate debt securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Corporate debt securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37,002	34,528
Foreign | Corporate debt securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Corporate debt securities | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37,002	34,528
Foreign | Real estate | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Real estate | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Real estate | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45,275	45,335
Foreign | Real estate | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	45,275	45,335
Foreign | Diversified growth funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Diversified growth funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Foreign | Diversified growth funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,750	47,621
Foreign | Diversified growth funds | Fair Value, Inputs, Level 1, 2 and 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	50,750	47,621
Foreign | Cash
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	6,448	1,516
Foreign | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 485	$ 484