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Significant Transactions (Dispositions) (Details) - USD ($)					3 Months Ended								9 Months Ended		12 Months Ended
	Feb. 26, 2019	Sep. 28, 2017	Jul. 21, 2016	Apr. 18, 2016	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2018	Sep. 28, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold															$ 0	$ 0	$ 1,131,000,000.0
Gain (loss) on sale of other discontinued operations, net of tax															800,000	(17,200,000)	557,000,000.0
Other comprehensive (loss) income, net of tax															(1,400,000)	(4,800,000)	300,000
Net income from discontinued operations, net of tax					$ (800,000)	$ 900,000	$ 0	$ 700,000	$ 0	$ (17,300,000)	$ 0	$ 100,000			0	0	20,500,000
Tranzact
Significant Transactions
Proceeds from dividends received			$ 221,300,000
MediaAlpha
Significant Transactions
Proceeds from dividends received	$ 89,300,000
Realized gain from transaction								67,500,000							67,500,000
Ownership interest (as a percent)	48.30%
Fully diluted ownership interest (as a percent)	42.00%
Assets, fair value								114,700,000
Unrealized investment gain								$ 114,700,000							180,000,000.0
Total realized and unrealized gain on sale of minority stake															182,200,000
OneBeacon
Significant Transactions
Sale of Stock, Price Per Share		$ 18.10												$ 18.10
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold		$ 1,300,000,000
Gain (loss) on sale of other discontinued operations, net of tax		554,600,000
Other comprehensive (loss) income, net of tax		$ 2,900,000
Net income from discontinued operations, net of tax														$ 20,500,000
Tranzact
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold			$ 221,300,000
Gain (loss) on sale of other discontinued operations, net of tax																3,200,000	3,200,000
Sirius Group
Significant Transactions
Sales of unconsolidated affiliates and consolidated subsidiaries, net of cash sold				$ 2,600,000,000
Gain (loss) on sale of other discontinued operations, net of tax																(17,300,000)
Disposal group, consideration				$ 2,600,000,000
Discontinued Operations | OneBeacon
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax																	554,500,000
Net income from discontinued operations, net of tax																	20,500,000
Discontinued Operations | Tranzact
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax																	3,200,000
Net income from discontinued operations, net of tax																	0
Discontinued Operations | Sirius Group
Significant Transactions
Gain (loss) on sale of other discontinued operations, net of tax															800,000	$ (17,200,000)	(700,000)
Net income from discontinued operations, net of tax																	$ 0
Swedish Tax Authority | Sirius Group | Sirius Group
Significant Transactions
Net income from discontinued operations, net of tax													$ (17,300,000)		$ 800,000
MediaAlpha
Significant Transactions
Ownership interest (as a percent)					48.30%										48.30%

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