CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCL, after-tax	Parent	Non-controlling interests	Total equity	Foreign Currency Gain (Loss) AOCL, after-tax	Foreign Currency Gain (Loss) Parent	Foreign Currency Gain (Loss) Non-controlling interests	Foreign Currency Gain (Loss) Total equity	Pension liability and other accumulated comprehensive items AOCL, after-tax	Pension liability and other accumulated comprehensive items Parent	Pension liability and other accumulated comprehensive items Total equity	Prospector Turtle Fund Non-controlling interests	Prospector Turtle Fund Total equity	Sale of Sirius Group	Sale of Sirius Group Non-controlling interests	Sale of Sirius Group Total equity	Tranzact Non-controlling interests	Tranzact Total equity	OneBeacon Non-controlling interests	OneBeacon Total equity	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCL, after-tax	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Parent	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total equity	Star & Shield LLC Non-controlling interests	Star & Shield LLC Total equity
Beginning balance at Dec. 31, 2014		$ 1,034.7	$ 3,003.0	$ (42.1)	$ 3,995.6	$ 542.6	$ 4,538.2
Increase (Decrease) in Shareholders' Equity
Net income	$ 276.7		295.2		295.2	(18.5)	276.7
Net change in pension liability and other accumulated comprehensive items								$ (73.2)	$ (73.2)		$ (73.2)
Other comprehensive income (loss), net of tax	(42.8)											$ (34.6)	$ (34.6)	$ (34.6)
Comprehensive income (loss)	187.4		295.2	(107.8)	187.4	(18.5)	168.9
Dividends declared on common shares			(6.0)		(6.0)		(6.0)
Dividends/distributions to non-controlling interests						(51.1)	(51.1)
Issuances of common shares		0.9			0.9		0.9
Repurchases and retirements of common shares	(284.2)	(67.0)	(217.2)		(284.2)		(284.2)
Net contributions from non-controlling interests						17.5	17.5
Acquisition of noncontrolling interest		(5.3)			(5.3)	(2.7)	(8.0)
Redemptions of the Prospector Turtle Fund															$ (31.5)	$ (31.5)
Amortization of restricted share and option awards		14.9			14.9	(2.0)	12.9
Ending balance at Dec. 31, 2015		978.2	3,075.0	(149.9)	3,903.3	454.3	4,357.6
Increase (Decrease) in Shareholders' Equity
Net income	409.0		401.8		401.8	7.2	409.0
Net change in pension liability and other accumulated comprehensive items								31.4	31.4	$ 0.3	31.7
Other comprehensive income (loss), net of tax	(0.7)											$ 113.9	$ 113.9	$ 113.9
Recognition of benefit plan assets and obligations from sale of OneBeacon																	$ 113.3
Comprehensive income (loss)	547.4		401.8	145.3	547.1	7.5	554.6
Dividends declared on common shares			(5.4)		(5.4)		(5.4)
Dividends/distributions to non-controlling interests						(22.7)	(22.7)
Issuances of common shares		9.1			9.1		9.1
Repurchases and retirements of common shares	(887.2)	(192.4)	(694.8)		(887.2)		(887.2)
Net contributions from non-controlling interests						27.3	27.3
Acquisition of noncontrolling interest		(2.7)			(2.7)	(8.8)	(11.5)
Acquisition of subsidiary						3.3	3.3
Deconsolidation of non-controlling interests associated with sale																		$ (250.0)	$ (250.0)	$ (78.4)	$ (78.4)
Amortization of restricted share and option awards		18.5			18.5	0.8	19.3
Ending balance at Dec. 31, 2016	3,582.7	810.7	2,776.6	(4.6)	3,582.7	133.3	3,716.0
Increase (Decrease) in Shareholders' Equity
Net income	593.1		627.2		627.2	(34.1)	593.1
Net change in pension liability and other accumulated comprehensive items								$ 0.4	$ 0.4	$ 0.1	$ 0.5
Other comprehensive income (loss), net of tax	0.3
Recognition of benefit plan assets and obligations from sale of OneBeacon																								$ 2.9	$ 2.9	$ 2.9
Comprehensive income (loss)	630.7		627.2	3.3	630.5	(34.0)	596.5
Dividends declared on common shares			(4.6)		(4.6)		(4.6)
Dividends/distributions to non-controlling interests						(19.3)	(19.3)
Issuances of common shares		1.7			1.7		1.7
Repurchases and retirements of common shares	(723.9)	(147.9)	(576.0)		(723.9)	(5.2)	(729.1)
Net contributions from non-controlling interests		(4.6)			(4.6)	30.2	25.6
Issuance of shares of non-controlling interests		(4.1)			(4.1)	5.2	1.1
Deconsolidation of non-controlling interests associated with sale																						$ (238.3)	$ (238.3)				$ (4.4)	$ (4.4)
Amortization of restricted share and option awards		14.8			14.8	0.8	15.6
Ending balance at Dec. 31, 2017	$ 3,492.5	$ 670.6	$ 2,823.2	$ (1.3)	$ 3,492.5	$ (131.7)	$ 3,360.8