Summary of Significant Accounting Policies (Details) (USD $) In Billions, unless otherwise specified	Jan. 01, 2009
Accounting Policies [Abstract]
Value assigned to equity interest to the acquired reporting entity	$ 29.1

Value assigned to equity interest to the acquired reporting entity

No definition available.

Name:	mer_ValueAssignedToEquityInterestToAcquiredReportingEntity
Namespace Prefix:	mer_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant