Business Segments - Additional Information (Detail) $ in Millions	12 Months Ended
Dec. 31, 2020 USD ($) Segment	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Segment Reporting Information [Line Items]
Operating segments \| Segment	2
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Market value of assets managed or held in custody by trust and investment services segment	$ 5,000.0	$ 4,700.0	$ 4,300.0
Total non-fiduciary assets of the trust and investment services segment	4.1	$ 4.2	$ 4.6
Trust and Investment Services [Member] \| Customer-Related Intangible Assets [Member]
Segment Reporting Information [Line Items]
Total non-fiduciary assets of the trust and investment services segment	$ 1.8

Number of operating segments. An operating segment is a component of an enterprise: (a) that engages in business activities from which it may earn revenues and incur expenses (including revenues and expenses relating to transactions with other components of the same enterprise), (b) whose operating results are regularly reviewed by the enterprise's chief operating decision maker to make decisions about resources to be allocated to the segment and assess its performance, and (c) for which discrete financial information is available. An operating segment may engage in business activities for which it has yet to earn revenues, for example, start-up operations may be operating segments before earning revenues.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=120311839&loc=d3e8672-108599

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