Derivatives - Reconciliation of Liabilities Classified as Level 3 (Detail) (Unobservable Inputs (Level 3) [Member], USD $) In Thousands, unless otherwise specified			3 Months Ended
Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Mark To Market On Series F Agreement [Member]
Derivative [Line Items]
Ending Liability Balance	$ (537)	$ (826)
Mark-to-Market of the Series F Agreement			289
Ending Liability Balance	$ (537)	$ (826)

This item represents the amount of the total realized and unrealized gains or losses for the period which are included in the statement of income (or changes in net assets); the fair value of which assets and liabilities was or is measured on a recurring basis using significant unobservable inputs (Level 3).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 820

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)(1)

-URI http://asc.fasb.org/extlink&oid=7578670&loc=d3e19207-110258

Name:	us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringBasisGainLossIncludedInEarnings
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration