Fair Value Measurements - Summary of Changes in Fair Value of Company's Level 3 Assets (Parenthetical) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Fair Value Measurements [Line Items]
Net unrealized gain (losses)	$ 12,910	$ (3,268)	$ 8,299
Sale/disposition of private equity fund investments		$ 49,500

Amount of unrealized gains and losses (net), included in the income statement for the period, from certain investments for which net asset value per share is calculated (including by unit, membership interest, or other equity (ownership interest) unit measure) (alternative investments) and which the entity has or had based its calculation of fair market value on 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_FairValueInvestmentsEntitiesThatCalculateNetAssetValuePerShareUnobservableInputUnrealizedGainsLossesIncludedInEarningsNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration