Segment Information - Additional Information (Detail)	12 Months Ended
Dec. 31, 2014 Segment	Dec. 31, 2013	Dec. 31, 2012
Segment Reporting Information [Line Items]
Number of reportable segments	2us-gaap_NumberOfReportableSegments
Number of customers individually accounting for more than ten percent of total operating revenues	No single customer accounted for more than 10% of our total operating revenues	No single customer accounted for more than 10% of our total operating revenues	No single customer accounted for more than 10% of our total operating revenues

Disclosure of the extent of enterprise reliance on its major customers. For example, includes revenues from transactions with a single external customer amounting to 10 percent or more of the entity's revenues, the total amount of revenues from each such customer, and the identity of the segment or segments reporting the revenues. A group of entities that the entity knows to be under common control generally will be considered a single customer for inclusion in this item. The federal government, a state government, a local government (for example, a county or municipality), or a foreign government each will generally be considered as a single customer for inclusion in this item.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 280

-SubTopic 10

-Section 50

-Paragraph 42

-URI http://asc.fasb.org/extlink&oid=6534315&loc=d3e9054-108599

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration