Retirement Plan - Additional Information (Detail) (USD $)	6 Months Ended	12 Months Ended
Jun. 30, 2012	Dec. 31, 2011
Employee Stock Ownership Plan (ESOP) Disclosures [Line Items]
Period during which the Company is required to purchase the shares at fair value after distribution of the shares from the ESOP, months	15 months
Distributed shares subject to put option, fair value	$ 136,233,000	$ 116,824,000
ESOP, shares cost	2,198,001,000	2,020,393,000
Common stock related to ESOP	2,334,234,000	2,137,217,000
ESOP shares, fair value	$ 5,587,805,000	$ 4,917,283,000

Fair value of the shares allocated that are subject to a repurchase obligation.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 40

-Section 50

-Paragraph 1

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=6418621&loc=d3e17540-113929

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

-Publisher AICPA

-Name Statement of Position (SOP)

-Number 93-6

-Paragraph 53

-Subparagraph f

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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

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