Business Combinations - Estimated Fair Values of Aggregate Assets and Liabilities Acquired (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	$ 27,673	$ 3,217	$ 11,054
Other current assets	22,043	12,956	52,917
Fixed assets	9,387	455	5,829
Goodwill	648,095	328,546	717,710
Other assets	3,652	55	18,588
Total assets acquired	952,024	473,943	1,151,328
Other current liabilities	(32,549)	(2,269)	(32,996)
Other liabilities	(26,313)	(407)	(565)
Deferred income tax, net	(344,222)	(328,277)
Total liabilities assumed	(58,862)	(2,676)	(33,561)
Net assets acquired	893,162	471,267	1,117,767
Special Risk
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	2,477
Fixed assets	345
Goodwill	63,087
Total assets acquired	80,331
Other current liabilities	(316)
Total liabilities assumed	(316)
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	27,845
Total assets acquired	38,496
Other current liabilities	(1,355)
Total liabilities assumed	(1,355)
Net assets acquired	37,141
RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets	55
Goodwill	53,567
Total assets acquired	66,412
Other current liabilities	(11,443)
Total liabilities assumed	(11,443)
Net assets acquired	54,969
Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	612
Fixed assets	16
Goodwill	17,339
Total assets acquired	23,950
Other current liabilities	(180)
Total liabilities assumed	(180)
Net assets acquired	23,770
LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	3,162
Fixed assets	1,877
Goodwill	99,983
Total assets acquired	149,854
Other current liabilities	(10)
Other liabilities	(184)
Total liabilities assumed	(194)
Net assets acquired	149,660
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	28,929
Other assets	274
Total assets acquired	44,842
Other current liabilities	(3,056)
Total liabilities assumed	(3,056)
Net assets acquired	41,786
BrookStone Insurance Group, LLC (BrookStone)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	527
Fixed assets	22
Goodwill	8,585
Other assets	290
Total assets acquired	13,134
Other current liabilities	(46)
Total liabilities assumed	(46)
Net assets acquired	13,088
South & Western
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets	149
Goodwill	63,128
Total assets acquired	81,811
Other current liabilities	(3,651)
Total liabilities assumed	(3,651)
Net assets acquired	78,160
Berry Insurance Group, Inc. (Berry)
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets	30
Goodwill	29,702
Total assets acquired	39,444
Other current liabilities	(424)
Total liabilities assumed	(424)
Net assets acquired	39,020
VAS
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash	27,673
Other current assets	5,486
Fixed assets	138
Goodwill	100,826
Total assets acquired	182,412
Other current liabilities	(3,760)
Other liabilities	(26,129)
Total liabilities assumed	(29,889)
Net assets acquired	152,523
Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	912	(6,786)	323
Fixed assets	25	(130)	100
Goodwill	11,325	34,314	22,712
Other assets	3,088	(732)	754
Total assets acquired	23,996	41,847	39,271
Other current liabilities	(120)	6,235	(1,715)
Total liabilities assumed	(120)	6,235	(1,715)
Net assets acquired	23,876	48,082	37,556
Bright & Associates, Inc. (Bright)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	402
Fixed assets	23
Goodwill	12,218
Total assets acquired	17,740
Other current liabilities	(101)
Total liabilities assumed	(101)
Net assets acquired	17,639
J.E. Brown
Fair Value Of Assets And Liabilities Statement [Line Items]
Fixed assets	32
Goodwill	31,476
Total assets acquired	41,028
Other current liabilities	(720)
Total liabilities assumed	(720)
Net assets acquired	40,308
CoverHound, Inc. and CyberPolicy, Inc. (CoverHound)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	375
Fixed assets	6,441
Goodwill	19,524
Total assets acquired	30,018
Other current liabilities	(1,823)
Total liabilities assumed	(1,823)
Net assets acquired	28,195
MAJ Companies, Ltd. (MAJ)
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets	413
Goodwill	13,003
Total assets acquired	21,461
Other current liabilities	(83)
Total liabilities assumed	(83)
Net assets acquired	21,378
Smith
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		680
Fixed assets		39
Goodwill		16,042
Total assets acquired		23,302
Other current liabilities		(469)
Total liabilities assumed		(469)
Net assets acquired		22,833
Pipino
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		819
Fixed assets		112
Goodwill		16,765
Other assets		772
Total assets acquired		29,839
Other current liabilities		(3,463)
Total liabilities assumed		(3,463)
Net assets acquired		26,376
Cossio
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		236
Fixed assets		29
Goodwill		10,010
Total assets acquired		14,699
Other current liabilities		(3)
Total liabilities assumed		(3)
Net assets acquired		14,696
Medval
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash		3,217
Other current assets		1,708
Fixed assets		50
Goodwill		19,108
Other assets		15
Total assets acquired		31,399
Other current liabilities		(480)
Other liabilities		(29)
Total liabilities assumed		(509)
Net assets acquired		30,890
United
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		477
Fixed assets		20
Goodwill		15,111
Total assets acquired		22,684
Other current liabilities		(41)
Total liabilities assumed		(41)
Net assets acquired		22,643
Twinbrook
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		919
Fixed assets		85
Goodwill		18,935
Total assets acquired		28,508
Other current liabilities		(292)
Total liabilities assumed		(292)
Net assets acquired		28,216
IRS
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		1,375
Fixed assets		11
Goodwill		24,938
Total assets acquired		35,135
Other current liabilities		(126)
Total liabilities assumed		(126)
Net assets acquired		35,009
WBR
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		449
Fixed assets		10
Goodwill		9,096
Total assets acquired		13,611
Other current liabilities		(166)
Other liabilities		(378)
Total liabilities assumed		(544)
Net assets acquired		13,067
Yozell
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		1,781
Fixed assets		12
Goodwill		8,904
Total assets acquired		14,268
Net assets acquired		14,268
CKP
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		9,170
Fixed assets		193
Goodwill		110,495
Total assets acquired		152,153
Other current liabilities		(870)
Total liabilities assumed		(870)
Net assets acquired		151,283
Poole
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		938
Fixed assets		4
Goodwill		28,233
Total assets acquired		39,567
Other current liabilities		(2,578)
Total liabilities assumed		(2,578)
Net assets acquired		36,989
VGW
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets		1,190
Fixed assets		20
Goodwill		16,595
Total assets acquired		26,931
Other current liabilities		(16)
Total liabilities assumed		(16)
Net assets acquired		26,915
Opus
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			1,215
Fixed assets			11
Goodwill			16,414
Other assets			315
Total assets acquired			22,984
Net assets acquired			22,984
Kerxton
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			663
Fixed assets			10
Goodwill			12,423
Other assets			419
Total assets acquired			18,249
Other current liabilities			(1,503)
Total liabilities assumed			(1,503)
Net assets acquired			16,746
ADG
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			1,500
Fixed assets			67
Goodwill			35,769
Other assets			467
Total assets acquired			47,575
Net assets acquired			47,575
Servco
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash			8,188
Other current assets			7,769
Fixed assets			179
Goodwill			54,429
Other assets			1,478
Total assets acquired			88,486
Other current liabilities			(11,307)
Total liabilities assumed			(11,307)
Net assets acquired			77,179
Tower Hill
Fair Value Of Assets And Liabilities Statement [Line Items]
Total assets acquired			21,488
Net assets acquired			21,488
HSR
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash			3,114
Other current assets			818
Fixed assets			124
Goodwill			18,737
Other assets			21
Total assets acquired			28,395
Other current liabilities			(5,930)
Deferred income tax, net			(342)
Total liabilities assumed			(6,272)
Net assets acquired			22,123
PDA
Fair Value Of Assets And Liabilities Statement [Line Items]
Cash			(248)
Other current assets			1,762
Fixed assets			310
Goodwill			16,547
Other assets			6
Total assets acquired			26,159
Other current liabilities			(1,093)
Deferred income tax, net			(223)
Total liabilities assumed			(1,316)
Net assets acquired			24,843
F&I
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			999
Fixed assets			34
Goodwill			36,423
Other assets			383
Total assets acquired			54,471
Net assets acquired			54,471
Rodman
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			1,062
Fixed assets			45
Goodwill			26,572
Other assets			542
Total assets acquired			38,401
Other current liabilities			(3,299)
Total liabilities assumed			(3,299)
Net assets acquired			35,102
Hays
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			36,254
Fixed assets			4,936
Goodwill			456,217
Other assets			13,977
Total assets acquired			732,584
Other current liabilities			(7,984)
Total liabilities assumed			(7,984)
Net assets acquired			724,600
Dealer Associates
Fair Value Of Assets And Liabilities Statement [Line Items]
Other current assets			552
Fixed assets			13
Goodwill			21,467
Other assets			226
Total assets acquired			33,265
Other current liabilities			(165)
Total liabilities assumed			(165)
Net assets acquired			33,100
Purchased Customer Accounts
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	239,961	128,302	342,008
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	9,205
Purchased Customer Accounts | RLA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	12,309
Purchased Customer Accounts | Sterling
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	5,962
Purchased Customer Accounts | LP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	44,801
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 Insurance Group, LLC (BrookStone)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	3,689
Purchased Customer Accounts | South & Western
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	18,513
Purchased Customer Accounts | Berry Insurance Group, Inc. (Berry)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	9,701
Purchased Customer Accounts | VAS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	48,188
Purchased Customer Accounts | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	8,582	15,020	15,085
Purchased Customer Accounts | Bright & Associates, Inc. (Bright)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	5,055
Purchased Customer Accounts | J.E. Brown
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	9,479
Purchased Customer Accounts | CoverHound, Inc. and CyberPolicy, Inc. (CoverHound)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	3,678
Purchased Customer Accounts | MAJ Companies, Ltd. (MAJ)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	8,034
Purchased Customer Accounts | Smith
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		6,500
Purchased Customer Accounts | Pipino
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		11,360
Purchased Customer Accounts | Cossio
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		4,403
Purchased Customer Accounts | Medval
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		7,300
Purchased Customer Accounts | United
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		7,065
Purchased Customer Accounts | Twinbrook
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		8,557
Purchased Customer Accounts | IRS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		8,800
Purchased Customer Accounts | WBR
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		4,022
Purchased Customer Accounts | Yozell
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		3,550
Purchased Customer Accounts | CKP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		32,274
Purchased Customer Accounts | Poole
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		10,359
Purchased Customer Accounts | VGW
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		9,092
Purchased Customer Accounts | Opus
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			5,008
Purchased Customer Accounts | Kerxton
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			4,712
Purchased Customer Accounts | ADG
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			9,751
Purchased Customer Accounts | Servco
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			16,442
Purchased Customer Accounts | Tower Hill
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			21,468
Purchased Customer Accounts | HSR
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			5,516
Purchased Customer Accounts | PDA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			7,700
Purchased Customer Accounts | F&I
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			16,611
Purchased Customer Accounts | Rodman
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			10,129
Purchased Customer Accounts | Hays
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			218,600
Purchased Customer Accounts | Dealer Associates
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			10,986
Non-compete Agreements
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	1,213	412	3,222
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 Insurance Group, LLC (BrookStone)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	21
Non-compete Agreements | South & Western
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	21
Non-compete Agreements | Berry Insurance Group, Inc. (Berry)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	11
Non-compete Agreements | VAS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	101
Non-compete Agreements | Other
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	64	161	297
Non-compete Agreements | Bright & Associates, Inc. (Bright)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	42
Non-compete Agreements | J.E. Brown
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	41
Non-compete Agreements | MAJ Companies, Ltd. (MAJ)
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets	$ 11
Non-compete Agreements | Smith
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		41
Non-compete Agreements | Pipino
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		11
Non-compete Agreements | Cossio
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		21
Non-compete Agreements | Medval
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		1
Non-compete Agreements | United
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		11
Non-compete Agreements | Twinbrook
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		12
Non-compete Agreements | IRS
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		11
Non-compete Agreements | WBR
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		34
Non-compete Agreements | Yozell
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		21
Non-compete Agreements | CKP
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		21
Non-compete Agreements | Poole
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		33
Non-compete Agreements | VGW
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets		$ 34
Non-compete Agreements | Opus
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			21
Non-compete Agreements | Kerxton
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			22
Non-compete Agreements | ADG
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			21
Non-compete Agreements | Servco
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			1
Non-compete Agreements | Tower Hill
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			20
Non-compete Agreements | HSR
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			65
Non-compete Agreements | PDA
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			82
Non-compete Agreements | F&I
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			21
Non-compete Agreements | Rodman
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			51
Non-compete Agreements | Hays
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			2,600
Non-compete Agreements | Dealer Associates
Fair Value Of Assets And Liabilities Statement [Line Items]
Intangible assets			$ 21