Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) $ in Thousands	Sep. 30, 2020 USD ($)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	$ 14,794
Fixed assets	2,550
Goodwill	378,633
Other assets	3,088
Total assets acquired	539,924
Other current liabilities	(21,251)
Total liabilities assumed	(21,251)
Net assets acquired	518,673
Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	1,702
Fixed assets	346
Goodwill	63,132
Total assets acquired	79,602
Other current liabilities	413
Total liabilities assumed	413
Net assets acquired	80,015
Texas Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	446
Fixed assets	27
Goodwill	8,940
Total assets acquired	12,660
Other current liabilities	(1,680)
Total liabilities assumed	(1,680)
Net assets acquired	10,980
Colonial
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	1,344
Fixed assets	59
Goodwill	25,719
Total assets acquired	36,558
Other current liabilities	(1,355)
Total liabilities assumed	(1,355)
Net assets acquired	35,203
RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets	54
Goodwill	51,285
Total assets acquired	64,695
Other current liabilities	(11,443)
Total liabilities assumed	(11,443)
Net assets acquired	53,252
Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	612
Fixed assets	16
Goodwill	14,816
Total assets acquired	22,661
Other current liabilities	(180)
Total liabilities assumed	(180)
Net assets acquired	22,481
LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	3,528
Fixed assets	1,877
Goodwill	115,200
Total assets acquired	169,269
Other current liabilities	(10)
Total liabilities assumed	(10)
Net assets acquired	169,259
First
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	302
Fixed assets	1
Goodwill	9,523
Total assets acquired	14,942
Other current liabilities	(16)
Total liabilities assumed	(16)
Net assets acquired	14,926
Buiten
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	2,595
Fixed assets	43
Goodwill	33,641
Total assets acquired	47,693
Other current liabilities	(845)
Total liabilities assumed	(845)
Net assets acquired	46,848
Amity
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	653
Fixed assets	58
Goodwill	15,454
Total assets acquired	21,800
Other current liabilities	(2,920)
Total liabilities assumed	(2,920)
Net assets acquired	18,880
Neal
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	2,337
Fixed assets	46
Goodwill	29,203
Total assets acquired	44,842
Other current liabilities	(3,056)
Total liabilities assumed	(3,056)
Net assets acquired	41,786
BrookStone
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	364
Fixed assets	23
Goodwill	9,031
Total assets acquired	13,128
Other current liabilities	(40)
Total liabilities assumed	(40)
Net assets acquired	13,088
Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	911
Goodwill	2,689
Other assets	3,088
Total assets acquired	12,074
Other current liabilities	(119)
Total liabilities assumed	(119)
Net assets acquired	11,955
Purchased Customer Accounts
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	139,885
Purchased Customer Accounts | Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	14,286
Purchased Customer Accounts | Texas Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	3,222
Purchased Customer Accounts | Colonial
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	9,393
Purchased Customer Accounts | RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	12,875
Purchased Customer Accounts | Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	7,196
Purchased Customer Accounts | LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	48,633
Purchased Customer Accounts | First
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	5,095
Purchased Customer Accounts | Buiten
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	11,323
Purchased Customer Accounts | Amity
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	5,614
Purchased Customer Accounts | Neal
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	13,225
Purchased Customer Accounts | BrookStone
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	3,689
Purchased Customer Accounts | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	5,334
Non-compete Agreements
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	974
Non-compete Agreements | Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	136
Non-compete Agreements | Texas Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	25
Non-compete Agreements | Colonial
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	43
Non-compete Agreements | RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	481
Non-compete Agreements | Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	21
Non-compete Agreements | LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	31
Non-compete Agreements | First
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	21
Non-compete Agreements | Buiten
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	91
Non-compete Agreements | Amity
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	21
Non-compete Agreements | Neal
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	31
Non-compete Agreements | BrookStone
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	21
Non-compete Agreements | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	$ 52