Acquisitions - Securities America Acquisition - Additional Information (Detail) (USD $) In Thousands	1 Months Ended	3 Months Ended
	Nov. 04, 2011	Mar. 31, 2012	Jun. 30, 2012	Dec. 31, 2011
Business Acquisition [Line Items]
Acquisition of outstanding capital stock of Securities America	$ 150,000
Earn out period	2 years
Business acquisition contingent consideration factor to determine earn out payment	70,000
Business acquisition date fair value of consideration transferred	157,111
Liability recognized for an estimate of the business acquisition earn out	7,111		909	7,111
Legal and other acquisition related costs	2,971
Decrease in estimated fair value of earn out			6,202
Deferred tax liability related to acquisition		$ (935)