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Employee Benefit Plans - Summary of Weighted-Average Assumptions Used to Determine Net Periodic Benefit Cost (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Pension Plan [Member]
Weighted-average assumptions used to determine Net Periodic Benefit Cost
Discount rate	3.59%	3.60%
Expected return on plan assets	7.00%	7.00%
Rate of compensation increase	0.00%	0.00%
Corridor	10.00%	10.00%
Supplemental Employee Retirement Plan [Member]
Weighted-average assumptions used to determine Net Periodic Benefit Cost
Discount rate	3.80%	3.40%
Rate of compensation increase	0.00%	0.00%
Corridor	10.00%	10.00%
Postretirement Benefits [Member]
Weighted-average assumptions used to determine Net Periodic Benefit Cost
Discount rate	4.00%	3.60%
Rate of compensation increase	0.00%	0.00%
Corridor	10.00%	10.00%

X

- Definition

Defined benefit plan assumptions used calculating net periodic benefit cost corridor.

+ References

No definition available.

+ Details

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X

- Definition

Weighted average rate for present value of future retirement benefits cash flows, used to determine net periodic benefit cost of defined benefit plan.

+ 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 (k)(1)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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X

- Definition

Weighted average rate of return on plan assets, reflecting average rate of earnings expected on existing plan assets and expected contributions, used to determine net periodic benefit cost of defined benefit plan.

+ 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 (k)(3)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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X

- Definition

Weighted average rate of compensation increase used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, pay-related defined benefit plan.

+ 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 (k)(2)
-URI http://asc.fasb.org/extlink&oid=82843861&loc=d3e1928-114920

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- References

No definition available.

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- Details

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- Details

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