Business Segments - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Segment	Dec. 31, 2018 USD ($)	Dec. 31, 2017 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	$ 4,700.0	$ 4,300.0	$ 3,900.0
Total non-fiduciary assets of the trust and investment services segment	4.2	$ 4.6	$ 1.5
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	$ 2.6