Off-Balance-Sheet Financial Instruments - Narrative (Detail) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
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	$ 62,000,000	$ 59,000,000
Indemnification Agreement
Fair Value, Off-balance Sheet Risks, Disclosure Information [Line Items]
Securities loaned and subject to indemnification	143,600,000,000	$ 102,300,000,000
Credit loss liability	$ 0