Schedule II - Condensed Financial Information of Parent - Derivatives (Detail) (Parent Issuer, USD $) In Millions, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Parent Issuer
Derivative [Line Items]
Collateral delivered	$ 23.3	$ 30.6

The carrying value as of the balance sheet date of securities received as collateral from a counterparty that were re-pledged, sold, delivered or otherwise reused by the entity.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 860

-SubTopic 30

-Section 50

-Paragraph 1A

-Subparagraph (c)(2)

-URI http://asc.fasb.org/extlink&oid=7523297&loc=SL6224234-111729

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