Long-Term Debt (Long-Term Debt - Additional Information) (Detail) - Senior Debt Obligations $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
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 Month LIBOR 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

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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=64930739&loc=d3e28878-108400

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=64930785&loc=d3e28551-108399

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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

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-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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- Definition

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

+ References

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