Stock-Based Compensation (Summary of Activity Relating to Restricted Units) (Parenthetical) (Detail) (USD $)	Dec. 31, 2012
Share-based Compensation [Abstract]
Closing market value of common stock	$ 22.32

Price of a single share of a number of saleable stocks of a company.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (e)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Name:	us-gaap_SharePrice
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant