Retirement Plans and Postretirement Medical Benefits (Benefit Obligations and Funded Status) (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
United States
Fair value of plan assets
Fair value of plan assets - beginning of year	$ 1,464,082
Fair value of plan assets - end of year	1,557,907	$ 1,464,082
Foreign
Fair value of plan assets
Fair value of plan assets - beginning of year	547,290
Fair value of plan assets - end of year	632,710	547,290
Pension Benefits \| United States
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	1,726,824	1,677,288
Projected benefit obligation
Benefit obligation - beginning of year	1,678,097	1,689,885
Service cost	132	105	$ 134
Interest cost	68,611	73,699	74,331
Plan participants' contributions	0	0
Actuarial loss	92,789	31,764
Foreign currency changes	0	0
Settlement	0	(5,887)
Benefits paid	(111,892)	(111,469)
Benefit obligation - end of year	1,727,737	1,678,097	1,689,885
Fair value of plan assets
Fair value of plan assets - beginning of year	1,464,082	1,460,790
Actual return on plan assets	199,749	110,954
Company contributions	5,968	9,694
Plan participants' contributions	0	0
Settlement	0	(5,887)
Foreign currency changes	0	0
Benefits paid	(111,892)	(111,469)
Fair value of plan assets - end of year	1,557,907	1,464,082	1,460,790
Amounts recognized in the Consolidated Balance Sheets
Noncurrent asset	392	310
Current liability	(8,362)	(7,937)
Noncurrent liability	(161,860)	(206,388)
Funded status	(169,830)	(214,015)
Pension Benefits \| Foreign
Defined Benefit Plan Disclosure [Line Items]
Accumulated benefit obligation	737,580	675,566
Projected benefit obligation
Benefit obligation - beginning of year	688,172	647,112
Service cost	2,274	2,148	2,229
Interest cost	18,836	21,886	24,261
Plan participants' contributions	6	6
Actuarial loss	2,098	127,054
Foreign currency changes	64,236	(88,138)
Settlement	0	(423)
Benefits paid	(24,249)	(21,473)
Benefit obligation - end of year	751,373	688,172	647,112
Fair value of plan assets
Fair value of plan assets - beginning of year	547,290	530,112
Actual return on plan assets	46,542	68,067
Company contributions	13,081	40,872
Plan participants' contributions	6	6
Settlement	0	(423)
Foreign currency changes	50,040	(69,871)
Benefits paid	(24,249)	(21,473)
Fair value of plan assets - end of year	632,710	547,290	530,112
Amounts recognized in the Consolidated Balance Sheets
Noncurrent asset	19,139	11,744
Current liability	(1,188)	(1,045)
Noncurrent liability	(136,614)	(151,581)
Funded status	(118,663)	(140,882)
Nonpension postretirement benefit plans
Projected benefit obligation
Benefit obligation - beginning of year	189,772	211,878
Service cost	1,727	2,046	2,455
Interest cost	7,100	7,969	8,799
Plan participants' contributions	3,820	4,241
Actuarial loss	5,134	(13,934)
Foreign currency changes	1,066	409
Benefits paid	(19,778)	(22,837)
Benefit obligation - end of year	188,841	189,772	211,878
Fair value of plan assets
Fair value of plan assets - beginning of year	0	0
Company contributions	15,958	18,596
Plan participants' contributions	3,820	4,241
Benefits paid	(19,778)	(22,837)
Fair value of plan assets - end of year	0	0	$ 0
Amounts recognized in the Consolidated Balance Sheets
Current liability	(17,712)	(18,127)
Noncurrent liability	(171,129)	(171,645)
Funded status	(188,841)	(189,772)
Nonpension postretirement benefit plans \| United States
Projected benefit obligation
Benefit obligation - beginning of year	174,000
Benefit obligation - end of year	$ 172,000	$ 174,000

Amount of actuarial present value of benefits attributed to employee service rendered, excluding assumptions about future compensation level.

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Amount of increase (decrease) in plan assets of defined benefit plan from actual return (loss) determined by change in fair value of plan assets adjusted for contributions, benefit payments, and other expenses.

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Amount of asset (liability), recognized in statement of financial position, for defined benefit pension and other postretirement plans.

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Amount of asset, recognized in statement of financial position, for overfunded defined benefit pension and other postretirement plans.

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Amount of actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment to participant of defined benefit plan which decreases benefit obligation. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Fair value of assets that have been segregated and restricted to provide pension or postretirement benefits. Assets include, but are not limited to, stocks, bonds, other investments, earnings from investments, and contributions by the employer and employees.

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Amount of foreign currency translation gain (loss) which (increases) decreases benefit obligation of defined benefit plan.

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Amount of cost recognized for passage of time related to defined benefit plan.

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Amount of payment to participant under defined benefit plan which decreases plan assets. For pension plan, payment includes, but is not limited to, pension benefits and death benefits. For other postretirement plan, payment includes, but is not limited to, prescription drug benefits, health care benefits, life insurance benefits, and legal, educational and advisory services.

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Amount of cost for actuarial present value of benefits attributed to service rendered by employee for defined benefit plan.

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Amount of payment, which decreases plan assets of defined benefit plan, for irrevocable action relieving primary responsibility for benefit obligation and eliminating risk for obligation and assets used to effect settlement. Transaction constituting settlement includes, but is not limited to, making lump-sum cash payment to participant in exchange for their rights to receive specified benefits and purchasing nonparticipating annuity contract.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as current.

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Amount of liability, recognized in statement of financial position, for defined benefit pension and other postretirement plans, classified as noncurrent.

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