Employee Benefit Plans and Equity Compensation Plan (Schedule of Assumptions Used in Determining Net Periodic Benefit Cost) (Detail)	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]
Ultimate health care cost trend rate	5.00%
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	3.47%		3.71%
Expected return on plan assets	7.00%	7.00%	7.00%
Rate of compensation increase	3.00%	3.00%	3.00%
Crediting interest rate for cash balance plans	2.67%		4.00%
Pension Benefits \| Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate		3.57%
Crediting interest rate for cash balance plans		2.77%
Pension Benefits \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate		4.38%
Crediting interest rate for cash balance plans		3.58%
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Discount rate	2.80%		3.74%
Health care cost trend rate	6.25%	6.60%	7.00%
Ultimate health care cost trend rate	5.00%	5.00%	5.00%
Year achieved		2023	2023
Other Postretirement Benefits \| Minimum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate		4.08%
Year achieved	2025
Other Postretirement Benefits \| Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Discount rate		4.41%
Year achieved	2026

Defined benefit plan assumptions used calculating net periodic benefit cost year achieved.

No definition available.

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- 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

+ Details

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- 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (k)(3)
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e1928-114920

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 715
-SubTopic 20
-Section 50
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=118255775&loc=d3e2919-114920

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- 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

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- Definition

Weighted-average interest crediting rate used to determine net periodic benefit cost of defined benefit plan. Plan includes, but is not limited to, cash balance and other defined benefit plans with promised interest crediting rate.

+ References

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- Definition

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.

+ References

No definition available.

+ Details

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- Definition

Assumed rate, for next fiscal year, based on annual change in cost of health care cost benefits used to measure expected cost of benefits covered by defined benefit postretirement plan. Factors include, but are not limited to, estimate of health care inflation, change in health care utilization or delivery pattern, technological advances, and change in health status of participant. Excludes factors for change in composition of plan population by age and dependency status.

+ References

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- Definition

Ultimate trend rate for health care cost for defined benefit postretirement plan.

+ References

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