Organization (Details) (Manager, USD $) In Millions, unless otherwise specified	Jun. 30, 2013
Manager
Organization
Offering and other related organization costs reimbursed	$ 1.2

Represents the threshold of organizational costs incurred by the entity of which any additional costs incurred are reimbursed by related party.

No definition available.

Name:	wmc_OrganizationalCostsReimbursedByRelatedPartyThreshold
Namespace Prefix:	wmc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant