Goodwill and Other Intangibles - Narrative (Detail) - USD ($) $ in Millions	1 Months Ended		3 Months Ended	12 Months Ended
	Sep. 30, 2019	Jun. 30, 2018	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization expense related to other intangible assets				$ 16.6	$ 17.4	$ 11.4
Goodwill				696.8	669.3	605.6
Other Intangible Assets
Acquired Finite-Lived Intangible Assets [Line Items]
Amortization expense related to other intangible assets				16.6	$ 17.4	$ 11.4
Estimated future amortization expense for 2020				16.7
Estimated future amortization expense for 2021				14.3
Estimated future amortization expense for 2022				9.7
Estimated future amortization expense for 2023				9.4
Estimated future amortization expense for 2024				9.3
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
Omnium Technology
Acquired Finite-Lived Intangible Assets [Line Items]
Purchase price		$ 73.0
Goodwill				71.4
Intangible assets acquired				$ 1.6