Financing Agreement - Additional Information (Detail) (USD $)		1 Months Ended
Aug. 31, 2011	Dec. 31, 2010 Master Notes [Member] Senior Unsecured Notes Three Point Eight One Percent Due May Twenty Eighteen [Member]	Dec. 31, 2010 Master Notes [Member] Senior Unsecured Notes Four Point Six Zero Percent Due May Twenty Twenty One [Member]	Dec. 31, 2013 Master Notes [Member] Senior Unsecured Notes Three Point One Three Percent Due December Twenty Twenty [Member]
Debt Instrument [Line Items]
Debt Instruments Maturity Date		5/31/2018	5/31/2021	12/31/2020
Proceeds from Issuance of Debt		$ 25,000,000	$ 75,000,000	$ 100,000,000
Line of Credit Facility, Maximum Borrowing Capacity	$ 350,000,000

Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 129

-Paragraph 2, 4

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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.

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

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