Fair Values of Financial Instruments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	6 Months Ended	12 Months Ended
Jun. 30, 2013	Dec. 31, 2012
Fair Value Measurements [Line Items]
Net unrealized loss on investment	$ 1.9	$ 2.4
Asset-backed securities and collateralized mortgage obligations
Fair Value Measurements [Line Items]
Asset-backed and mortgage-backed securities with maturities	26 years
Corporate notes and bonds
Fair Value Measurements [Line Items]
Asset-backed and mortgage-backed securities with maturities	3 years

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 230

-SubTopic 10

-Section 45

-Paragraph 28

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585

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