Fair Value Disclosures of Financial Assets - Assets Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance	$ 19,229	$ 15,722
Total net income (losses) for the period included in net income	734	42
Purchases, sales, issuances, and settlements, net	(179)	180
Mark-to-market adjustment	(1,239)	3,285
Balance	18,545	19,229
Trading Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance		12,432
Total net income (losses) for the period included in net income		33
Mark-to-market adjustment		125
Balance	12,590	12,590
Reverse Mortgages [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance	(457)	(646)
Total net income (losses) for the period included in net income	214	9
Purchases, sales, issuances, and settlements, net	(179)	180
Balance	(422)	(457)
Available-For-Sale Securities [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Balance	7,096	3,936
Total net income (losses) for the period included in net income	520
Mark-to-market adjustment	(1,239)	3,160
Balance	$ 6,377	$ 7,096