Significant Transactions (Acquisitions) (Details)								3 Months Ended	8 Months Ended	12 Months Ended
	Dec. 03, 2018 USD ($) shares	May 11, 2018 USD ($)	Jan. 24, 2018 USD ($)	Oct. 05, 2017 USD ($) shares	Aug. 01, 2017 USD ($) shares	Aug. 04, 2016 USD ($)	Jan. 15, 2016 USD ($)	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2018 USD ($) earnout_period	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	May 18, 2018 USD ($)	Feb. 05, 2018 USD ($)
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired										$ 295,200,000	$ 27,600,000	$ 13,400,000
Assets									$ 3,362,600,000	3,362,600,000	3,659,200,000
Goodwill									379,900,000	379,900,000	25,900,000	$ 25,900,000
Liabilities									644,400,000	644,400,000	298,400,000
NSM Holding Co LLC
Unusual or Infrequent Item, or Both [Line Items]
Contingent consideration									2,700,000	2,700,000
Contingent consideration earnout liabilities									20,200,000	20,200,000	0
Assets									627,000,000	627,000,000	0
Liabilities									$ 314,800,000	$ 314,800,000	$ 0
Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)						70.90%			77.10%	77.10%	77.10%
Assets						$ 11,500,000
Goodwill						7,600,000
Intangible assets, net (excluding goodwill)						1,100,000
Liabilities						100,000
LowFares.com
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired							$ 3,900,000
Healthplans.com
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired				$ 28,000,000
MediaAlpha
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									61.00%	61.00%	64.40%
Assets									$ 88,400,000	$ 88,400,000	$ 96,500,000
Liabilities									$ 46,900,000	$ 46,900,000	$ 59,800,000
Preferred stocks | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Stock issued during period, shares, acquisitions | shares					37,409
Common Stock | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Stock issued during period, shares, acquisitions | shares					5,808
GBP | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary						6,100,000
GBP | Preferred stocks | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					$ 4,000,000
GBP | Common Stock | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					500,000
United States of America, Dollars | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary						$ 8,100,000
United States of America, Dollars | Preferred stocks | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					5,000,000
United States of America, Dollars | Common Stock | Buzzmove
Unusual or Infrequent Item, or Both [Line Items]
Payments to acquire interest in subsidiary					$ 700,000
NSM Insurance HoldCo, LLC
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)		95.00%							95.50%	95.50%
Purchase of consolidated subsidiaries, net of cash acquired		$ 274,200,000.0
Purchase price adjustment								$ 2,100,000
Assets acquired		495,200,000
Goodwill and intangible assets acquired		383,000,000
Liabilities acquired		204,600,000
Contingent consideration		10,200,000
Noncontrolling interest acquired		$ 14,400,000
Transaction costs									$ 6,300,000
Fresh Insurance Services Group Limited
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)													100.00%
Purchase price adjustment								$ 700,000
Assets acquired													$ 72,600,000
Goodwill and intangible assets acquired													54,600,000
Liabilities acquired													22,300,000
Contingent consideration													7,500,000
Equity method investment, aggregate cost													$ 49,600,000
Number of earnout periods | earnout_period										2
KBK Insurance Group, Inc. & KBK Premium Services, Inc.
Unusual or Infrequent Item, or Both [Line Items]
Purchase of consolidated subsidiaries, net of cash acquired	$ 60,000,000.0
Goodwill and intangible assets acquired	$ 59,400,000.0
Stock issued during period, shares, acquisitions | shares	16,694,869
Stock issued during period, value, acquisitions	$ 29,000,000
DavidShield
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)			50.00%						50.00%	50.00%
DavidShield | White Mountains
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									25.00%	25.00%
PassportCard
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)			50.00%						25.00%	25.00%	50.00%
DavidShield & PassportCard
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									50.00%	50.00%
Payments to acquire businesses, gross			$ 41,800,000
Payments to acquire businesses, net			$ 28,300,000
Kudu
Unusual or Infrequent Item, or Both [Line Items]
Ownership interest (as a percent)									49.50%	49.50%	0.00%
Equity method investment, aggregate cost														$ 125,000,000
Working capital funded									$ 3,800,000	$ 3,800,000
Additional funding									63,000,000	63,000,000
Kudu | White Mountains
Unusual or Infrequent Item, or Both [Line Items]
Equity method investment, aggregate cost									31,500,000	31,500,000
Additional funding									$ 31,500,000	$ 31,500,000
Capital Unit, Class A | MediaAlpha
Unusual or Infrequent Item, or Both [Line Items]
Capital units, purchased | shares				131,579
Capital units, value				$ 12,500,000