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 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 2: 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