CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY Unaudited - USD ($) $ in Millions	Total	Common shares and paid-in surplus	Retained earnings	AOCI, after tax	Total	Non-controlling interest	Total Equity	Foreign Currency Gain (Loss) Common shares and paid-in surplus	Foreign Currency Gain (Loss) Retained earnings	Foreign Currency Gain (Loss) AOCI, after tax	Foreign Currency Gain (Loss) Total	Foreign Currency Gain (Loss) Non-controlling interest	Foreign Currency Gain (Loss) Total Equity	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling interest	Star & Shield LLC Common shares and paid-in surplus	Star & Shield LLC Retained earnings	Star & Shield LLC AOCI, after tax	Star & Shield LLC Total	Star & Shield LLC Non-controlling interest	Star & Shield LLC Total Equity	OneBeacon Non-controlling interest	OneBeacon Total Equity	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCI, after tax	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total	OneBeacon Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Common shares and paid-in surplus	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Retained earnings	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCI, after tax	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Non-controlling interest	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity	Sale of Sirius Group	Sale of Sirius Group Common shares and paid-in surplus	Sale of Sirius Group Retained earnings	Sale of Sirius Group AOCI, after tax	Sale of Sirius Group Total	Sale of Sirius Group Non-controlling interest	Sale of Sirius Group Total Equity	Sale of Tranzact Common shares and paid-in surplus	Sale of Tranzact Retained earnings	Sale of Tranzact AOCI, after tax	Sale of Tranzact Total	Sale of Tranzact Non-controlling interest	Sale of Tranzact Total Equity	OneBeacon Insurance Company Common shares and paid-in surplus	OneBeacon Insurance Company Retained earnings	OneBeacon Insurance Company AOCI, after tax	OneBeacon Insurance Company Total	OneBeacon Insurance Company Non-controlling interest	OneBeacon Insurance Company Total Equity
Ending Balances 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	$ 465.3	0.0	440.7	0.0	440.7	24.6	465.3
Net change in foreign currency translation and benefit plan assets and obligations	(0.4)							$ 0.0	$ 0.0	$ 32.1	$ 32.1	$ 0.1	$ 32.2
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary																												$ 0.0	$ 0.0	$ 113.3	$ 113.3	$ 0.0	$ 113.3
Total comprehensive income	586.1	0.0	440.7	145.4	586.1	24.7	610.8
Dividends declared on common shares		0.0	(5.4)	0.0	(5.4)	0.0	(5.4)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(17.4)	(17.4)
Repurchases and retirements of common shares		(188.1)	(678.6)	0.0	(866.7)	0.0	(866.7)
Issuance of common shares		9.1	0.0	0.0	9.1	0.0	9.1
Deconsolidation of non-controlling interests																																			$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (250.0)	$ (250.0)	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (78.4)	$ (78.4)
Acquisition of subsidiary		0.0	0.0	0.0	0.0	3.4	3.4
Non-controlling interests, decrease from purchase of interests																																															$ (2.7)	$ 0.0	$ 0.0	$ (2.7)	$ (8.8)	$ (11.5)
Issuance of shares to non-controlling interests		0.0	0.0	0.0	0.0	0.3	0.3
Net contributions from non-controlling interests		0.0	0.0	0.0	0.0	20.1	20.1
Amortization of restricted share awards		13.3	0.0	0.0	13.3	0.5	13.8
Ending Balances at Sep. 30, 2016		809.8	2,831.7	(4.5)	3,637.0	148.7	3,785.7
Beginning Balances at Dec. 31, 2015		978.2	3,075.0	(149.9)	3,903.3	454.3	4,357.6
Increase (Decrease) in Shareholders' Equity
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary																																		$ 113.3
Ending Balances at Dec. 31, 2016	3,716.0	810.7	2,776.6	(4.6)	3,582.7	133.3	3,716.0
Beginning Balances at Dec. 31, 2016	3,716.0	810.7	2,776.6	(4.6)	3,582.7	133.3	3,716.0
Increase (Decrease) in Shareholders' Equity
Net income	581.1	0.0	604.7	0.0	604.7	(23.6)	581.1
Net change in foreign currency translation and benefit plan assets and obligations	0.2							$ 0.0	$ 0.0	$ 0.4	$ 0.4	$ 0.1	$ 0.5
Recognition of foreign currency translation and other accumulated comprehensive items from the sale of subsidiary														$ 0.0	$ 0.0	$ 0.0									$ 2.9	$ 2.9	$ 2.9
Total comprehensive income	608.1	0.0	604.7	3.3	608.0	(23.5)	584.5
Dividends declared on common shares		0.0	(4.6)	0.0	(4.6)	0.0	(4.6)
Dividends to non-controlling interests		0.0	0.0	0.0	0.0	(17.7)	(17.7)
Repurchases and retirements of common shares		(147.9)	(575.9)	0.0	(723.8)	(5.2)	(729.0)
Issuance of common shares		1.7	0.0	0.0	1.7	0.0	1.7
Deconsolidation of non-controlling interests																	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (4.4)	$ (4.4)	$ (238.3)	$ (238.3)
Non-controlling interests, decrease from purchase of interests		0.0	0.0	0.0	0.0	0.0	0.0
Issuance of shares to non-controlling interests		(4.1)	0.0	0.0	(4.1)	4.7	0.6
Net contributions from non-controlling interests		(1.7)	0.0	0.0	(1.7)	18.7	17.0
Amortization of restricted share awards		10.6	0.0	0.0	10.6	0.8	11.4
Ending Balances at Sep. 30, 2017	3,337.2	669.3	2,800.8	(1.3)	3,468.8	(131.6)	3,337.2
Increase (Decrease) in Shareholders' Equity
Net income	551.5
Net change in foreign currency translation and benefit plan assets and obligations	0.0
Total comprehensive income	565.1
Ending Balances at Sep. 30, 2017	$ 3,337.2	$ 669.3	$ 2,800.8	$ (1.3)	$ 3,468.8	$ (131.6)	$ 3,337.2