CONSOLIDATED STATEMENTS OF CHANGES IN EQUITY - 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	Star & Shield LLC [Member] Common shares and paid-in surplus	Star & Shield LLC [Member] Retained earnings	Star & Shield LLC [Member] AOCI, after tax	Star & Shield LLC [Member] Total	Star & Shield LLC [Member] Non-controlling interest	Star & Shield LLC [Member] Total Equity
Balances at Dec. 31, 2015		$ 978.2	$ 3,084.9	$ (149.9)	$ 3,913.2	$ 454.8	$ 4,368.0
Increase (Decrease) in Shareholders' Equity
Net income	$ 19.3	0.0	13.0	0.0	13.0	6.3	19.3
Net change in foreign currency translation	0.1							$ 0.0	$ 0.0	$ 37.3	$ 37.3	$ 0.0	$ 37.3
Total comprehensive income		0.0	13.0	37.3	50.3	6.3	56.6
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	(6.2)	(6.2)
Repurchases and retirements of common shares		(39.8)	(132.9)	0.0	(172.7)	0.0	(172.7)
Acquisition from non-controlling interests - OneBeacon		(2.7)	0.0	0.0	(2.7)	(8.8)	(11.5)
Net contributions from non-controlling interests		0.0	0.0	0.0	0.0	0.3	0.3
Noncontrolling Interest, Period Increase (Decrease)		0.0	0.0	0.0	0.0	4.8	4.8
Amortization of restricted share awards		3.2	0.0	0.0	3.2	0.2	3.4
Balances at Mar. 31, 2016		938.9	2,959.6	(112.6)	3,785.9	451.4	4,237.3
Balances at Dec. 31, 2016	3,737.2	810.7	2,797.2	(4.6)	3,603.3	133.9	3,737.2
Increase (Decrease) in Shareholders' Equity
Net income	33.0	0.0	34.3	0.0	34.3	(1.3)	33.0
Net change in foreign currency translation	0.9							$ 0.0	$ 0.0	$ 0.9	$ 0.9	$ 0.0	$ 0.9
Total comprehensive income		0.0	34.3	0.9	35.2	(1.3)	33.9
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	(6.6)	(6.6)
Repurchases and retirements of common shares		(1.4)	(5.1)	0.0	(6.5)	(1.1)	(7.6)
Deconsolidation of non-controlling interests associated with the sale of Star & Shield														$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (4.4)	$ (4.4)
Net contributions from non-controlling interests		(4.8)	0.0	0.0	(4.8)	4.8	0.0
Noncontrolling Interest, Period Increase (Decrease)		0.0	0.0	0.0	0.0	6.9	6.9
Amortization of restricted share awards		2.6	0.0	0.0	2.6	0.2	2.8
Balances at Mar. 31, 2017	$ 3,757.6	$ 807.1	$ 2,821.8	$ (3.7)	$ 3,625.2	$ 132.4	$ 3,757.6