Goodwill and Other Intangibles - Narrative (Detail) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Acquired Finite-Lived Intangible Assets [Line Items]
Other Intangibles Amortization			$ 16.9	$ 16.6	$ 17.4
Goodwill			707.2	696.8	669.3
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Other Intangibles Amortization			16.9	16.6	17.4
Estimated future amortization expense for 2021			15.1
Estimated future amortization expense for 2022			10.5
Estimated future amortization expense for 2023			10.2
Estimated future amortization expense for 2024			10.1
Estimated future amortization expense for 2025			9.5
Computer Software
Acquired Finite-Lived Intangible Assets [Line Items]
Other Intangibles Amortization			$ 366.9	$ 339.1	$ 334.9
Belvedere Advisors LLC
Acquired Finite-Lived Intangible Assets [Line Items]
Purchase price	$ 17.6
Goodwill	9.3
Belvedere Advisors LLC | Computer Software
Acquired Finite-Lived Intangible Assets [Line Items]
Intangible assets acquired	$ 8.3
BEx LLC
Acquired Finite-Lived Intangible Assets [Line Items]
Purchase price		$ 37.9
Goodwill		12.5
Intangible assets acquired		$ 25.0