Transactions - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	1 Months Ended
May 31, 2012	Mar. 31, 2012
Business Acquisition [Line Items]
Purchase price for acquisition of OneSteel Piping Systems		$ 87.9
Payment related to the working capital	$ 15.1

Business acquisition purchase price, including working capital adjustments.

No definition available.

Name:	mrc_BusinessAcquisitionPurchasePrice
Namespace Prefix:	mrc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant