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Significant Transactions - MediaAlpha (Details) - USD ($) $ / shares in Units, $ in Millions			3 Months Ended
	Mar. 23, 2021	Oct. 30, 2020	Dec. 31, 2020	Mar. 31, 2021
Secondary Offering | Marketing Technology (MediaAlpha)
Other Significant Noncash Transactions [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)
Other Significant Noncash Transactions [Line Items]
Proceeds from dividends received		$ 55.0	$ 55.0
Investment owned (in shares)	16,900,000	20,500,000
Ownership interest (as a percent)	28.50%	35.00%
Fully diluted ownership interest (as a percent)	26.30%	32.30%
Sale of stock, price per share, net of underwriting fees (in USD per share)		$ 17.67
Share price (in USD per share)		$ 19.00	$ 39.07	$ 35.43
Investment owned, balance			$ 802.2	$ 600.2
Marketing Technology (MediaAlpha) | IPO
Other Significant Noncash Transactions [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
Other Significant Noncash Transactions [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

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