Retirement Plans and Other Postretirement Benefits - Schedule of Net Benefit Cost (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Pension Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	$ 13,669	$ 12,630	$ 27,429	$ 26,896
Interest cost on benefit obligation	32,177	32,878	64,878	65,823
Expected return on plan assets	(43,425)	(43,161)	(87,135)	(86,953)
Amortization of:
Prior service cost (credit)	20	132	41	263
Net actuarial loss	11,460	10,627	23,950	20,358
Net periodic benefit cost	13,901	13,106	29,163	26,387
Portion of cost charged to expense	6,894	6,433	14,461	12,951
Other Benefits
Retirement Plans and Other Benefits
Service cost — benefits earned during the period	4,201	3,560	8,559	7,497
Interest cost on benefit obligation	7,415	7,519	14,980	14,860
Expected return on plan assets	(13,350)	(9,125)	(26,701)	(18,247)
Amortization of:
Prior service cost (credit)	(9,461)	(9,471)	(18,921)	(18,942)
Net actuarial loss	1,104	1,349	2,559	2,295
Net periodic benefit cost	(10,091)	(6,168)	(19,524)	(12,537)
Portion of cost charged to expense	$ (5,004)	$ (3,027)	$ (9,682)	$ (6,153)

Amortization of Prior Service Cost and Net Actuarial Loss [Abstract]

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The amount of gains or losses recognized in net periodic benefit cost.

+ References

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- Definition

The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (h)(1)
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-Publisher FASB
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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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No definition available.

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An amount calculated as a basis for determining the extent of delayed recognition of the effects of changes in the fair value of assets. The expected return on plan assets is determined based on the expected long-term rate of return on plan assets and the market-related value of plan assets.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=6512171

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- Definition

The increase in a defined benefit pension plan's projected benefit obligation or a defined benefit postretirement plan's accumulated postretirement benefit obligation due to the passage of time.

+ References

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-Publisher FASB
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- Definition

The total amount of net periodic benefit cost for defined benefit plans for the period. Periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss), prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments.

+ References

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- Definition

The actuarial present value of benefits attributed by the pension benefit formula to services rendered by employees during the period. The portion of the expected postretirement benefit obligation attributed to employee service during the period. The service cost component is a portion of the benefit obligation and is unaffected by the funded status of the plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Service Cost (Component of Net Periodic Pension Cost)
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The amount of pension and other (such as medical, dental and life insurance) postretirement benefit costs recognized during the period for (1) defined benefit plans (periodic benefit costs include the following components: service cost, interest cost, expected return on plan assets, gain (loss) on assets, prior service cost or credit, transition asset or obligation, and gain (loss) due to settlements or curtailments) and for (2) defined contribution plans (to the extent that a plan's defined contributions to an individual's account are to be made for periods in which that individual renders services, the net cost for a period is the contribution called for in that period; if a plan calls for contributions for periods after an individual retires or terminates, the estimated cost is accrued during the employee's service period).

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