RCap Regulatory Requirements - Narrative (Detail) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015
RCAP Regulatory Requirements:
Minimum net capital requirement	$ 0.3
Regulatory net capital	399.9nly_RegulatoryNetCapital
Regulatory net capital, excess net capital	$ 399.6nly_ExcessNetCapital

Minimum amount of net capital required to be maintained by entity's broker-dealer subsidiary to engage in securities transactions.

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

-Publisher SEC

-Name Rule 15c3

-Number 240

-Section 1

-Paragraph 1

-Subparagraph i

-Article 15c3

-Subsection a

Name:	us-gaap_MinimumNetCapitalRequiredForBrokerDealerSubsidiary1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant