Long-Term Debt (Long-Term Debt - Additional Information) (Detail) - Senior Debt Obligations $ in Millions	9 Months Ended
Sep. 30, 2018 USD ($)
Debt Instrument
Face amount of notes issued or redeemed	$ 1,550
Fixed Rate 3.95 Percent Senior Notes Due 2028
Debt Instrument
Face amount of notes issued or redeemed	$ 650
Maturity date(s) Start	Mar. 14, 2018
Maturity date(s) End	Mar. 14, 2028
Floating Rate 3 ML Plus 47 bps Notes Due 2021
Debt Instrument
Face amount of notes issued or redeemed	$ 250
Maturity date(s) Start	Jun. 05, 2018
Maturity date(s) End	Jun. 04, 2021
Fixed Rate 3.35 Percent Senior Notes Due 2021
Debt Instrument
Face amount of notes issued or redeemed	$ 500
Maturity date(s) Start	Jul. 26, 2018
Maturity date(s) End	Jul. 26, 2021
Floating Rate 3 ML Plus 44 bps Senior Notes Due 2021
Debt Instrument
Face amount of notes issued or redeemed	$ 300
Maturity date(s) Start	Jul. 26, 2018
Maturity date(s) End	Jul. 26, 2021
Fixed Rate 3.95 Percent Senior Notes Due 2025
Debt Instrument
Face amount of notes issued or redeemed	$ 750
Maturity date(s) Start	Jul. 26, 2018
Maturity date(s) End	Jul. 28, 2025

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

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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Latest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

+ References

Reference 1: 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)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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Earliest date the outstanding debt instruments are required to be repaid, in CCYY-MM-DD format.

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