Goodwill and Other Acquisition-Related Intangible Assets - Other Acquisition-Related Intangibles (Detail) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	$ 536.5	$ 455.5
Accumulated Amortization	327.4	292.0
Carrying Amount	209.1	163.5
Total other acquisition-related intangible assets	209.1	180.0
Mutual Fund Management Contracts [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Indefinite-lived intangible assets	0.0	16.5
Core Deposit Intangibles [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	303.9	222.9
Accumulated Amortization	177.3	157.4
Carrying Amount	126.6	65.5
Trade Name [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	123.9	123.9
Accumulated Amortization	73.4	64.8
Carrying Amount	50.5	59.1
Client Relationship [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	24.4	24.4
Accumulated Amortization	6.4	4.4
Carrying Amount	18.0	20.0
Customer Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	42.7	42.7
Accumulated Amortization	35.1	31.3
Carrying Amount	7.6	11.4
Insurance Relationships [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	38.1	38.1
Accumulated Amortization	34.3	33.6
Carrying Amount	3.8	4.5
Favorable Lease Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	2.9	2.9
Accumulated Amortization	0.6	0.3
Carrying Amount	2.3	2.6
Noncompete Agreements [Member]
Acquired Finite-Lived Intangible Assets [Line Items]
Gross Amount	0.6	0.6
Accumulated Amortization	0.3	0.2
Carrying Amount	$ 0.3	$ 0.4