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Equity-Method Eligible Investments - Narrative (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	Mar. 23, 2021	Oct. 30, 2020	Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Schedule of Equity Method Investments [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha			$ (40.6)		$ (156.7)
Marketing Technology (MediaAlpha)
Schedule of Equity Method Investments [Line Items]
Net realized and unrealized investment (losses) gains from investment in MediaAlpha			$ (41.7)		30.0
Secondary Offering | Marketing Technology (MediaAlpha)
Schedule of Equity Method Investments [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	8,050,000.00
Sale of stock, price per share (in USD per share)	$ 46.00	$ 19.00
Sale of stock, price per share, net of underwriting fees (in USD per share)	$ 44.62	$ 17.67
Marketing Technology (MediaAlpha)
Schedule of Equity Method Investments [Line Items]
Proceeds from dividends received		$ 55.0		$ 55.0
Ownership interest (as a percent)	28.50%	35.00%
Investment owned (in shares)	16,900,000	20,500,000
Fully diluted ownership interest (as a percent)	26.30%	32.30%
Share price (in USD per share)		$ 19.00	$ 35.43	$ 39.07
Investment owned, balance			$ 600.2	$ 802.2
Sale of stock, price per share, net of underwriting fees (in USD per share)		$ 17.67
Marketing Technology (MediaAlpha) | IPO
Schedule of Equity Method Investments [Line Items]
Sale of stock, number of shares issued in transaction (in shares)		3,600,000
Sale of stock, consideration received in transaction		$ 63.8
Investment owned (in shares)		20,500,000
Marketing Technology (MediaAlpha) | Secondary Offering
Schedule of Equity Method Investments [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	3,600,000
Sale of stock, consideration received in transaction	$ 160.3
Investment owned (in shares)	16,900,000
Marketing Technology (MediaAlpha)
Schedule of Equity Method Investments [Line Items]
Proceeds from dividends received			$ 11.4		13.7
Ownership interest (as a percent)			28.50%	35.00%
Net realized and unrealized investment (losses) gains from investment in MediaAlpha			$ (41.7)		$ 30.0

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