Acquisitions and Dispositions - Narrative (Detail)			6 Months Ended		8 Months Ended	12 Months Ended
	Jul. 01, 2020 USD ($) earn_out_payment	Sep. 04, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	May 06, 2019 USD ($)
Business Acquisition [Line Items]
HKFS contingent consideration			$ 35,900,000	$ 35,900,000		$ 35,900,000
Goodwill			454,821,000	454,821,000	$ 662,375,000	454,821,000	$ 662,375,000	$ 548,685,000
Increase of goodwill						(666,000)
Acquisition and integration						31,085,000	25,763,000	0
Proceeds from sale of business, net of cash divested						349,000	7,467,000	0
Gain on sale of investment							3,300,000	$ 2,100,000
SimpleTax
Business Acquisition [Line Items]
Proceeds from sale of business, net of cash divested		$ 9,600,000
Senior Secured Credit Facility
Business Acquisition [Line Items]
Credit facility borrowed			563,156,000	563,156,000	399,687,000	563,156,000	399,687,000
Term Loan | Senior Secured Credit Facility
Business Acquisition [Line Items]
Aggregate principal amount	$ 175,000,000.0								$ 125,000,000.0
Credit facility borrowed			675,000,000.0	675,000,000.0		675,000,000.0			125,000,000.0
Honkamp Krueger Financial Services, Inc.
Business Acquisition [Line Items]
Business combination purchase price	$ 104,400,000
Number of potential earn-out payments | earn_out_payment	2
Undiscounted contingent consideration			30,000,000.0	30,000,000.0		30,000,000.0
HKFS contingent consideration	$ 27,600,000		27,600,000	27,600,000		27,600,000
Goodwill	58,137,000		63,737,000	63,737,000		63,737,000
Identifiable intangible assets, adjustment				(5,600,000)
Increase of goodwill			5,600,000	5,600,000
Acquisition related costs	$ 10,800,000					7,700,000	3,100,000
Revenue from acquiree since acquisition date						19,600,000
Earnings of acquiree since acquisition date						4,500,000
Acquisition and integration						19,700,000	3,100,000
Honkamp Krueger Financial Services, Inc. | Customer-Related Intangible Assets
Business Acquisition [Line Items]
Identifiable intangible assets, adjustment			(5,600,000)
1st Global
Business Acquisition [Line Items]
Goodwill					117,126,000		117,126,000		117,792,000
Identifiable intangible assets, adjustment					0
Increase of goodwill					(666,000)
Acquisition related costs							6,500,000
Revenue from acquiree since acquisition date					114,800,000
Earnings of acquiree since acquisition date					300,000
Contingent liability			$ 11,300,000	$ 11,300,000	$ 11,052,000	11,300,000	11,052,000		$ 11,052,000
Acquisition and integration						$ 11,400,000	$ 22,700,000