Fair Value of Financial Instruments - Summary of Carrying Values and Fair Values of Financial Instruments (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017
Financial assets
Investment securities			$ 908,297
Loans receivable			864,819
Investments			43,478
Carrying Amount [Member] | Bank Holding Company Accounting [Member]
Financial assets
Cash and federal funds sold	[1]	$ 143,560
Equity investments		10,752
Investment securities		45,757
Loans receivable		1,060,061
Accrued interest receivable	[2]	7,005
Financial liabilities
Funds borrowed	[3]	1,265,074
Accrued interest payable		6,118
Carrying Amount [Member] | Investment Company Accounting [Member]
Financial assets
Cash and federal funds sold	[1]		12,690
Investments			610,135
Accrued interest receivable	[2]		547
Financial liabilities
Funds borrowed	[3]		327,623
Accrued interest payable			3,831
Fair Value Recurring [Member]
Financial assets
Investments	[4]	45,757
Fair Value Recurring [Member] | Bank Holding Company Accounting [Member]
Financial assets
Cash and federal funds sold	[1]	143,560
Equity investments		10,752
Investment securities		45,757
Loans receivable		1,060,061
Accrued interest receivable	[2]	7,005
Financial liabilities
Funds borrowed	[3]	1,266,016
Accrued interest payable		$ 6,118
Fair Value Recurring [Member] | Investment Company Accounting [Member]
Financial assets
Cash and federal funds sold	[1]		12,690
Investments			610,135
Accrued interest receivable	[2]		547
Financial liabilities
Funds borrowed	[3]		330,084
Accrued interest payable			$ 3,831

[1] Categorized as level 1 within the fair value hierarchy.

[2] Categorized as level 3 within the fair value hierarchy.

[3] As of September 30, 2018 and December 31, 2017, publicly traded retail notes traded at a premium to par of $942 and $2,461.

[4] Total unrealized losses of $469, net of tax, was included in accumulated other comprehensive income (loss) for the six months ended September 30, 2018 related to these assets