Equity-Method Eligible Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended								10 Months Ended	12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Oct. 30, 2020	Feb. 26, 2019	Feb. 25, 2019
Schedule of Equity Method Investments [Line Items]
Net investment income										$ 131.0	$ 79.7	$ 59.0
Unrealized investment gain											180.0
Net realized and unrealized investment gains from MediaAlpha										716.8	433.2	(108.3)
Total revenues	$ 520.8	$ 459.5	$ 259.6	$ (59.2)	$ 175.3	$ 155.5	$ 129.0	$ 433.6		1,180.7	893.4	369.1
Other Operations
Schedule of Equity Method Investments [Line Items]
Net investment income										82.0	43.4	42.3
Net realized and unrealized investment gains from MediaAlpha												0.0
Total revenues										$ 781.4	523.7	(53.9)
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)													35.00%
Fully diluted ownership interest (as a percent)													32.30%
Investment owned (in shares)													20,532,202
Share price (in USD per share)	$ 39.07									$ 39.07
Investment owned, balance	$ 802.2									$ 802.2
MediaAlpha
Schedule of Equity Method Investments [Line Items]
Net investment income										$ 95.0	$ 14.8	3.7
Ownership interest (as a percent)	35.00%				48.30%				48.30%	35.00%	48.30%
Assets, fair value								114.7
Unrealized investment gain								$ 114.7
Net realized and unrealized investment gains from MediaAlpha										$ 686.0	$ 180.0	$ 0.0
Total revenues									$ 359.2
Total expenses									336.3
Net income									$ 22.9
MediaAlpha | Other Operations
Schedule of Equity Method Investments [Line Items]
Net realized and unrealized investment gains from MediaAlpha										$ 686.0	$ 180.0
MediaAlpha | Minimum
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)														48.30%
Fully diluted ownership interest (as a percent)														42.00%
MediaAlpha | Maximum
Schedule of Equity Method Investments [Line Items]
Ownership interest (as a percent)															61.00%
Fully diluted ownership interest (as a percent)															58.90%