Business Combinations - Schedule of Business Acquisitions (Details) - USD ($) $ in Thousands | Apr. 01, 2020 | Sep. 30, 2020 | Sep. 30, 2019 | Jun. 01, 2020 | Jan. 01, 2020 |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | $ 238,118 | | | |
Fair value of contingent earnout consideration | | 26,612 | $ 29,117 | | |
Fair value of equity interest | | 52,575 | $ 38,637 | | |
Business Combination, Consideration Transferred, Liabilities Incurred | | 359 | | | |
Total consideration | | 317,664 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | | 7,715 | | | |
Premiums, commissions and fees receivable | | 26,835 | | | |
Property and equipment | | 443 | | | |
Other assets | | 222 | | | |
Goodwill | | 179,926 | | | |
Total assets acquired | | 338,775 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | | (19,717) | | | |
Accrued expenses and other current liabilities | | (1,394) | | | |
Total liabilities acquired | | (21,111) | | | |
Net assets acquired | | 317,664 | | | |
Maximum potential contingent earnout consideration (1) | | 134,769 | | | |
Lanier | | | | | |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | 24,450 | | | |
Fair value of contingent earnout consideration | | 1,628 | | | |
Fair value of equity interest | | 6,119 | | | |
Total consideration | | 32,197 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | | | | | $ 2,413 |
Premiums, commissions and fees receivable | | | | | 2,494 |
Property and equipment | | | | | 294 |
Other assets | | | | | 168 |
Goodwill | | | | | 25,735 |
Total assets acquired | | | | | 35,416 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | | | | | (2,954) |
Accrued expenses and other current liabilities | | | | | (265) |
Total liabilities acquired | | | | | (3,219) |
Net assets acquired | | | | | 32,197 |
Maximum potential contingent earnout consideration (1) | | | | | 11,000 |
Highland | | | | | |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | 6,603 | | | |
Fair value of contingent earnout consideration | | 788 | | | |
Fair value of equity interest | | 4,500 | | | |
Total consideration | | 11,891 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | | | | | 1,542 |
Premiums, commissions and fees receivable | | | | | 5,929 |
Property and equipment | | | | | 0 |
Other assets | | | | | 13 |
Goodwill | | | | | 4,276 |
Total assets acquired | | | | | 19,133 |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | | | | | (6,374) |
Accrued expenses and other current liabilities | | | | | (868) |
Total liabilities acquired | | | | | (7,242) |
Net assets acquired | | | | | 11,891 |
Maximum potential contingent earnout consideration (1) | | | | | 2,450 |
IRP | | | | | |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | 26,223 | | | |
Fair value of contingent earnout consideration | | 5,521 | | | |
Fair value of equity interest | | 7,535 | | | |
Total consideration | | 39,279 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | $ 1,694 | | | | |
Premiums, commissions and fees receivable | 3,229 | | | | |
Property and equipment | 123 | | | | |
Other assets | 11 | | | | |
Goodwill | 32,273 | | | | |
Total assets acquired | 43,580 | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | (4,301) | | | | |
Accrued expenses and other current liabilities | 0 | | | | |
Total liabilities acquired | (4,301) | | | | |
Net assets acquired | 39,279 | | | | |
Maximum potential contingent earnout consideration (1) | $ 44,399 | | | | |
Compound annual revenue growth rate | 50.00% | | | | |
Period to which compound annual revenue growth rate has been applied | 3 years | | | | |
Rosenthal | | | | | |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | 75,936 | | | |
Fair value of contingent earnout consideration | | 7,051 | | | |
Fair value of equity interest | | 10,147 | | | |
Total consideration | | 93,134 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | | | | $ 954 | |
Premiums, commissions and fees receivable | | | | 4,734 | |
Property and equipment | | | | 0 | |
Other assets | | | | 11 | |
Goodwill | | | | 56,253 | |
Total assets acquired | | | | 96,561 | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | | | | (3,427) | |
Accrued expenses and other current liabilities | | | | 0 | |
Total liabilities acquired | | | | (3,427) | |
Net assets acquired | | | | 93,134 | |
Maximum potential contingent earnout consideration (1) | | | | 30,843 | |
TBA/RBA | | | | | |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | 76,272 | | | |
Fair value of contingent earnout consideration | | 7,000 | | | |
Fair value of equity interest | | 21,427 | | | |
Business Combination, Consideration Transferred, Liabilities Incurred | | 300 | | | |
Total consideration | | 104,999 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | | | | 0 | |
Premiums, commissions and fees receivable | | | | 8,731 | |
Property and equipment | | | | 0 | |
Other assets | | | | 0 | |
Goodwill | | | | 44,264 | |
Total assets acquired | | | | 107,074 | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | | | | (2,075) | |
Accrued expenses and other current liabilities | | | | 0 | |
Total liabilities acquired | | | | (2,075) | |
Net assets acquired | | | | 104,999 | |
Maximum potential contingent earnout consideration (1) | | | | 30,147 | |
All Others | | | | | |
Business Combination, Consideration Transferred [Abstract] | | | | | |
Cash consideration paid | | 28,634 | | | |
Fair value of contingent earnout consideration | | 4,624 | | | |
Fair value of equity interest | | 2,847 | | | |
Business Combination, Consideration Transferred, Liabilities Incurred | | 59 | | | |
Total consideration | | 36,164 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Cash | | 1,112 | | | |
Premiums, commissions and fees receivable | | 1,718 | | | |
Property and equipment | | 26 | | | |
Other assets | | 19 | | | |
Goodwill | | 17,125 | | | |
Total assets acquired | | 37,011 | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Liabilities [Abstract] | | | | | |
Premiums and producer commissions payable | | (586) | | | |
Accrued expenses and other current liabilities | | (261) | | | |
Total liabilities acquired | | (847) | | | |
Net assets acquired | | 36,164 | | | |
Maximum potential contingent earnout consideration (1) | | 15,930 | | | |
Purchased customer accounts | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 101,621 | | | |
Purchased customer accounts | Lanier | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 4,312 |
Purchased customer accounts | Highland | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 0 |
Purchased customer accounts | IRP | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | $ 6,250 | | | | |
Purchased customer accounts | Rosenthal | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 33,670 | |
Purchased customer accounts | TBA/RBA | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 53,900 | |
Purchased customer accounts | All Others | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 3,489 | | | |
Distributor relationships | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 19,380 | | | |
Distributor relationships | Lanier | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 0 |
Distributor relationships | Highland | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 6,500 |
Distributor relationships | IRP | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | 0 | | | | |
Distributor relationships | Rosenthal | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 0 | |
Distributor relationships | TBA/RBA | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 0 | |
Distributor relationships | All Others | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 12,880 | | | |
Carrier relationships | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 659 | | | |
Carrier relationships | Lanier | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 0 |
Carrier relationships | Highland | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 659 |
Carrier relationships | IRP | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | 0 | | | | |
Carrier relationships | Rosenthal | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 0 | |
Carrier relationships | TBA/RBA | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 0 | |
Carrier relationships | All Others | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 0 | | | |
Software | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 565 | | | |
Software | Lanier | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 0 |
Software | Highland | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 0 |
Software | IRP | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | 0 | | | | |
Software | Rosenthal | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 0 | |
Software | TBA/RBA | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 0 | |
Software | All Others | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 565 | | | |
Trade names | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | 1,409 | | | |
Trade names | Lanier | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | 0 |
Trade names | Highland | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | | $ 214 |
Trade names | IRP | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | $ 0 | | | | |
Trade names | Rosenthal | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | 939 | |
Trade names | TBA/RBA | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | | | $ 179 | |
Trade names | All Others | | | | | |
Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets [Abstract] | | | | | |
Intangible assets | | $ 77 | | | |