Off Balance Sheet Financial Instruments - Additional Information (Detail) - USD ($)	Dec. 31, 2016	Dec. 31, 2015
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Minimum collateral maintained against fair value of client securities loaned plus accrued interest	100.00%
Estimated credit exposure from clearing activities	$ 59,000,000	$ 58,000,000
Indemnification Agreement
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Securities loaned and subject to indemnification	102,300,000,000	$ 94,500,000,000
Credit loss liability	$ 0