Principal Amount Effective Interest Rate and Maturity Date for Debentures and Senior Notes (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012
6.6% Senior Notes, Due 2018
Debt Instrument
Principal Amount	$ 300,000
Effective Interest Rate	6.81%
Maturity Date	April 15, 2018
7% Debentures, Due 2025
Debt Instrument
Principal Amount	125,000
Effective Interest Rate	7.12%
Maturity Date	December 1, 2025
6.25% Senior Notes, Due 2037
Debt Instrument
Principal Amount	$ 230,000
Effective Interest Rate	6.45%
Maturity Date	May 1, 2037

The stated principal amount of the debt instrument at time of issuance, which may vary from the carrying amount because of unamortized premium or discount.

Reference 1: 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=6451184&loc=d3e28551-108399

Reference 2: 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=6584090&loc=d3e28878-108400

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher AICPA

-Name Accounting Principles Board Opinion (APB)

-Number 21

-Paragraph 16, 20

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Name:	us-gaap_DebtInstrumentFaceAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant