Business Segments - Additional Information (Detail) $ in Millions |
3 Months Ended | |
|---|---|---|
|
Mar. 31, 2018
USD ($)
Segment
|
Mar. 31, 2017
USD ($)
|
|
| 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 | $ 4,000.0 | $ 3,800.0 |
| Total non-fiduciary assets of the trust and investment services segment | $ 1.8 | $ 3.8 |