Goodwill and Other Intangible Assets - Narrative (Details) - USD ($)				6 Months Ended
	Jun. 28, 2019	Dec. 03, 2018	May 11, 2018	Jun. 30, 2019	Jun. 30, 2018	Apr. 04, 2019	Apr. 01, 2019	Feb. 26, 2019	Dec. 31, 2018	May 18, 2018
Goodwill [Line Items]
Purchases of consolidated subsidiaries, net of cash acquired				$ 243,400,000	$ 252,100,000
NSM Insurance HoldCo, LLC
Goodwill [Line Items]
Goodwill and intangible assets acquired			$ 383,000,000.0
Purchases of consolidated subsidiaries, net of cash acquired			$ 274,200,000
Ownership interest (as a percent)			95.00%	96.40%					95.50%
Fully diluted ownership interest (as a percent)				88.40%					85.00%
Fresh Insurance Services Group Limited
Goodwill [Line Items]
Goodwill and intangible assets acquired										$ 54,600,000
Ownership interest (as a percent)										100.00%
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Goodwill [Line Items]
Goodwill and intangible assets acquired		$ 59,400,000		$ 32,600,000
Intangible assets acquired				$ 32,700,000
Purchases of consolidated subsidiaries, net of cash acquired		$ 60,000,000.0
AIG Collector Car Business
Goodwill [Line Items]
Intangible assets acquired	$ 82,500,000
Purchases of consolidated subsidiaries, net of cash acquired	$ 82,500,000
Acquired finite-lived intangible assets, weighted average useful life (in years)	12 years
MediaAlpha
Goodwill [Line Items]
Ownership interest (as a percent)								48.30%
Fully diluted ownership interest (as a percent)								42.00%
Embrace Pet Insurance | NSM Holding Co LLC
Goodwill [Line Items]
Goodwill and intangible assets acquired							$ 70,600,000
Kudu | White Mountains
Goodwill [Line Items]
Goodwill and intangible assets acquired						$ 9,800,000