Commitments and Contingencies - Additional Information (Detail)	6 Months Ended
Jun. 30, 2012 Year	Nov. 11, 2009 Plaintiff
Commitments and Contingencies Disclosure [Line Items]
Participant's retirement age	65
Treasury rate, years	30
Named plaintiffs		2
Number of defendants	200
Unasserted Claim
Commitments and Contingencies Disclosure [Line Items]
Number of claimed properties	1,000

The total number of new claims filed pertaining to a loss contingency during the period.

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 5

-Paragraph 9, 10, 11, 12

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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_LossContingencyNewClaimsFiledNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration