Securities (Securities - Additional Information) (Detail) - USD ($) $ in Billions	Sep. 30, 2018	Dec. 31, 2017
Investment Holdings
Securities with a fair value, pledged as collateral	$ 6.3	$ 7.8

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_InvestmentHoldingsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration