Notes Payable, Warrants and Subordinated Borrowings - Additional Information (Detail) (USD $) Share data in Thousands, except Per Share data, unless otherwise specified	12 Months Ended	0 Months Ended
	Dec. 31, 2014	Aug. 21, 2008	Oct. 31, 2014	Dec. 31, 2013
Notes Payable And Warrants [Line Items]
Senior Notes	$ 120,000,000
Subordinated Borrowings	22,550,000		22,550,000	0
Subordinated Borrowing, Interest Rate	5.50%
Class A [Member]
Notes Payable And Warrants [Line Items]
Maximum percent of voting securities and total shares of class A common stock owned by holder or group of affiliates for warrant transfer (as a percent)	2.00%
Class A [Member] | Maximum [Member]
Notes Payable And Warrants [Line Items]
Percent of outstanding shares of common stock owned by holder or group of affiliates for warrant transfer (as a percent)	25.00%
Senior Notes [Member]
Notes Payable And Warrants [Line Items]
Senior Notes		$ 120,000,000
Principal amount of senior notes due year	2020
Senior Notes coupon interest (as a percent)		5.20%
Senior notes effective yield (as a percent)	7.94%
Redemption period (in days)	90 days
Event of default under the indenture accreted amount due and payable (as a percent)	33.33%
Maximum percent of aggregate principal amount of senior notes owned by holder or group of affiliates for transfer (as a percent)	15.00%
Warrant [Member] | Class A [Member]
Notes Payable And Warrants [Line Items]
Warrants to purchase Class A shares (in shares)		5,455
Class A Shares purchase price per share (in dollars per share)		$ 22.00