Long-Term Debt - Schedule of Principal Amount, Effective Interest Rate and Maturity Date for Corporation's Senior Notes (Detail) - USD ($)	12 Months Ended
Dec. 31, 2019	Dec. 31, 2018
4.25% Senior Notes, Due 2024
Debt Instrument [Line Items]
Principal Amount	$ 400,000,000.0
Effective Interest Rate	4.25%
Maturity Date	Jul. 02, 2024
7% Debentures, Due 2025
Debt Instrument [Line Items]
Principal Amount	$ 125,000,000.0
Effective Interest Rate	7.12%
Maturity Date	Dec. 01, 2025
3.450% Senior Notes, Due 2027
Debt Instrument [Line Items]
Principal Amount	$ 300,000,000.0
Effective Interest Rate	3.47%
Maturity Date	Jun. 01, 2027
3.500% Senior Notes, Due 2027
Debt Instrument [Line Items]
Principal Amount	$ 500,000,000.0
Effective Interest Rate	3.53%
Maturity Date	Dec. 15, 2027
6.25% Senior Notes, Due 2037
Debt Instrument [Line Items]
Principal Amount	$ 230,000,000.0
Effective Interest Rate	6.45%
Maturity Date	May 01, 2037
4.250% Senior Notes, Due 2047
Debt Instrument [Line Items]
Principal Amount	$ 600,000,000.0
Effective Interest Rate	4.27%
Maturity Date	Dec. 15, 2047
Floating Rate Senior Notes, Due 2020
Debt Instrument [Line Items]
Principal Amount	$ 300,000,000.0
Effective Interest Rate	2.55%	3.30%
Debt Instrument Effective Interest Rate	Three-month LIBOR + 0.65%
Maturity Date	May 22, 2020

Debt instrument effective interest rate.

No definition available.

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- Definition

Face (par) amount of debt instrument at time of issuance.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

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- Definition

Effective interest rate for the funds borrowed under the debt agreement considering interest compounding and original issue discount or premium.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 6
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6036836-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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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.

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- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (bbb)(2)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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