Notes Payable and Subordinated Borrowings - Additional Information (Detail) - USD ($)		1 Months Ended	6 Months Ended
	Mar. 30, 2016	Apr. 30, 2016	Jun. 30, 2016	Dec. 31, 2015
Notes Payable [Line Items]
Minimum Repayment Of Aggregate Principal Amount Of Senior Notes (as a percent)			5.00%
Outstanding Principal Amount Of Senior Notes (as a percent)			100.00%
Subordinated Borrowing, Interest Rate (as a percent)			5.50%
Subordinated Borrowings			$ 16,550,000	$ 22,550,000
Senior Notes [Member]
Notes Payable [Line Items]
Long-term Debt, Gross	$ 170,000,000
Series A Senior Notes [Member]
Notes Payable [Line Items]
Long-term Debt, Gross	$ 38,000,000
Debt Instrument, Interest Rate, Stated Percentage (as a percent)	4.88%
Series B Senior Notes [Member]
Notes Payable [Line Items]
Long-term Debt, Gross	$ 67,000,000
Debt Instrument, Interest Rate, Stated Percentage (as a percent)	5.23%
Series C Senior Notes [Member]
Notes Payable [Line Items]
Long-term Debt, Gross	$ 48,000,000
Debt Instrument, Interest Rate, Stated Percentage (as a percent)	5.48%
Series D Senior Notes [Member]
Notes Payable [Line Items]
Long-term Debt, Gross	$ 17,000,000
Debt Instrument, Interest Rate, Stated Percentage (as a percent)	5.58%
Senior Notes [Member]
Notes Payable [Line Items]
Extinguishment of Debt, Amount	$ 120,000,000
Subordinated Debt [Member]
Notes Payable [Line Items]
Extinguishment of Debt, Amount		$ 6,000,000