Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Millions	Oct. 30, 2020	Oct. 26, 2020	Oct. 01, 2020	Oct. 31, 2020	Sep. 30, 2020	Dec. 31, 2019	Feb. 26, 2019	Dec. 31, 2018
Subsequent Event [Line Items]
Other long-term investments					$ 1,171.2	$ 856.3
MediaAlpha
Subsequent Event [Line Items]
Fully diluted ownership interest (as a percent)							42.00%	58.90%
Subsequent Event | Ark Insurance Holdings Limited
Subsequent Event [Line Items]
Merger agreement, equity capital contributed			$ 605.4
Equity capital, pre money valuation amount			300.0
Additional equity purchase from shareholders			40.9
Investment, potential additional contribution of equity capital			$ 200.0
Ownership interest after all transactions			72.00%
Business combination, consideration transferred			$ 800.0
Investment owned, after additional equity contributions			$ 1,000.0
Fully diluted ownership interest after additional funding			63.00%
Equity incentives, rate of return threshold			8.00%
Percent of shares outstanding			12.50%
Amount of equity contribution pre-funded		$ 400.0
Subsequent Event | Ark Insurance Holdings Limited | Tranche One
Subsequent Event [Line Items]
Invested capital return threshold, multiples			2.0
Subsequent Event | Ark Insurance Holdings Limited | Tranche Two
Subsequent Event [Line Items]
Invested capital return threshold, multiples			2.5
Subsequent Event | Ark Insurance Holdings Limited | Tranche Three
Subsequent Event [Line Items]
Invested capital return threshold, multiples			3.0
Subsequent Event | Ark Insurance Holdings Limited | Forecast
Subsequent Event [Line Items]
Ownership interest after additional funding			77.10%
Fully diluted ownership interest after additional funding			67.50%
Subsequent Event | MediaAlpha
Subsequent Event [Line Items]
Share price (in USD per share)	$ 33.83
Subsequent Event | Ark Insurance Holdings Limited
Subsequent Event [Line Items]
Letters of credit outstanding		$ 150.0	$ 150.0
Subsequent Event | IPO | MediaAlpha
Subsequent Event [Line Items]
Sale of stock, price per share (in USD per share)	$ 19.00
Equity market capitalization	$ 1,200.0
Equity enterprise valuation	1,400.0
MediaAlpha | Subsequent Event
Subsequent Event [Line Items]
Other long-term investments	$ 390.1			$ 694.6
Investment owned (in shares)	20,532,202
Fully diluted ownership interest (as a percent)	32.30%
MediaAlpha | Subsequent Event | IPO
Subsequent Event [Line Items]
Sale of stock, number of shares issued in transaction (in shares)	3,609,894
Sale of stock, consideration received in transaction	$ 63.8