Subsequent Events - Additional Information (Detail) (USD $)	1 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended	3 Months Ended	12 Months Ended
Feb. 19, 2013 Subsequent Event [Member]	Dec. 31, 2012 Chief Financial Officer [Member] Subsequent Event [Member]	Nov. 08, 2012 Class A common stock [Member]	Jun. 26, 2012 Class A common stock [Member]	Dec. 31, 2012 Class A common stock [Member] Subsequent Event [Member]	Nov. 08, 2012 Class B common stock [Member]	Jun. 26, 2012 Class B common stock [Member]	Dec. 31, 2012 Class B common stock [Member] Subsequent Event [Member]
Subsequent Event [Line Items]
Dividend declared, record date for shareholders	Feb. 19, 2013				Mar. 20, 2013			Mar. 20, 2013
Common Stock Dividends Per Share Declared			$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25	$ 0.25
Resignation Date of Daniel D Greenwell Chief executive officer		Mar. 31, 2013

Date the dividend to be paid was declared, in CCYY-MM-DD format.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 50

-Paragraph 3

-URI http://asc.fasb.org/extlink&oid=6367179&loc=d3e4304-108586

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 95

-Paragraph 32

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009.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:dateItemType
Balance Type:	na
Period Type:	duration