Financial Instruments - Summary of Bond Activity (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
Sep. 30, 2013	Dec. 31, 2012
Derivative Instruments And Hedging Activities Disclosure [Abstract]
Fair value at beginning of period	$ 29,776	$ 30,923
Purchases	4,965	5,742
Sales	(345)	(2,000)
Maturities and calls	(3,800)	(4,900)
Decrease in bond premium	(253)	(290)
Fair market value adjustment	(217)	301
Fair value at end of period	$ 30,126	$ 29,776

The fair value as of the balance sheet date of firm holdings in fixed income or other debt securities issued by a corporation or other issuer not elsewhere specified in the taxonomy. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=29643736&loc=d3e42546-110969

Name:	us-gaap_FinancialInstrumentsOwnedCorporateDebtAtFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant