Schedule II - Condensed Financial Information of Registrant - Statements of (Loss) Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Revenues
Net investment income (losses)									$ 71.4	$ 56.8	$ 56.2
Net realized investment (losses) gains									2.3	(27.2)	288.3
Net unrealized investment gains (losses)									(23.2)	(10.5)	(238.2)
Net foreign exchange (losses)									22.7	9.2	(11.0)
Gain on revaluation of contingent consideration									9.6	48.8
Other revenue									112.7	21.7	9.1
Total revenues	$ 336.8	$ 351.7	$ 441.8	$ 327.5	$ 362.6	$ 293.0	$ 252.6	$ 225.9	1,457.8	1,134.1	994.5
Expenses
Other underwriting expenses									146.2	106.1	107.3
General and administrative expenses									77.9	91.9	85.1
Total expenses	482.3	386.3	292.4	273.1	305.8	478.1	240.2	219.9	1,434.1	1,244.0	956.6
Income (loss) before equity earnings of unconsolidated affiliates, net of tax									(16.7)	(136.3)	45.2
Equity in earnings of unconsolidated affiliates, net of tax											6.6
Net (loss) income attributable to Sirius Group									(18.1)	(150.0)	32.5
Less: Accrued dividends on Series A redeemable preference shares									(2.6)	(6.1)
Less: Change in carrying value of Series B preference shares									(36.4)
Add: Redemption of Series A redeemable preference shares									13.8
Net (loss) income attributable to Sirius Group's common shareholders	$ (153.6)	$ (28.0)	$ 97.8	$ 40.5	$ 16.0	$ (179.0)	$ (0.8)	$ 7.7	(43.3)	(156.1)	32.5
Registrant
Revenues
Net investment income (losses)									0.5	0.3	(0.2)
Net realized investment (losses) gains										0.2	0.3
Net unrealized investment gains (losses)										12.8	(0.1)
Net foreign exchange (losses)										(0.2)
Gain on revaluation of contingent consideration									6.1	13.6
Other revenue									41.4	0.9
Total revenues									48.0	27.6
Expenses
Other underwriting expenses										(0.2)
General and administrative expenses									13.6	13.0	36.6
Total expenses									13.6	12.8	36.6
Income (loss) before equity earnings of unconsolidated affiliates, net of tax									34.4	14.8	(36.6)
Equity in earnings of unconsolidated affiliates, net of tax									(52.5)	(164.8)	69.1
Net (loss) income attributable to Sirius Group									(18.1)	(150.0)	32.5
Less: Accrued dividends on Series A redeemable preference shares									(2.6)	(6.1)
Less: Change in carrying value of Series B preference shares									(36.4)
Add: Redemption of Series A redeemable preference shares									13.8
Net (loss) income attributable to Sirius Group's common shareholders									$ (43.3)	$ (156.1)	$ 32.5