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 includes Noncontrolling Interest [Member]	Foreign Currency Gain (Loss) Common shares and paid-in surplus	Foreign Currency Gain (Loss) Retained earnings	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 includes Noncontrolling Interest [Member]	Pension liability and other accumulated comprehensive items Common shares and paid-in surplus	Pension liability and other accumulated comprehensive items Retained earnings	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 Non-controlling Interests	Pension liability and other accumulated comprehensive items Total Equity includes Noncontrolling Interest [Member]	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 Interests	Recognition of foreign currency translation and other accumulated comprehensive items [Member] Total Equity includes Noncontrolling Interest [Member]	Prospector Turtle Fund Common shares and paid-in surplus	Prospector Turtle Fund Retained earnings	Prospector Turtle Fund AOCL, after-tax	Prospector Turtle Fund Parent	Prospector Turtle Fund Non-controlling Interests	Prospector Turtle Fund Total Equity includes Noncontrolling Interest [Member]	Buzzmove [Member] Common shares and paid-in surplus	Buzzmove [Member] Retained earnings	Buzzmove [Member] AOCL, after-tax	Buzzmove [Member] Parent	Buzzmove [Member] Non-controlling Interests	Tranzact Non-controlling Interests	Sirius Group Non-controlling Interests	Sirius Group Total Equity includes Noncontrolling Interest [Member]	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] AOCL, after-tax	Sirius Group Recognition of foreign currency translation and other accumulated comprehensive items [Member] Parent	Sale of Sirius Group [Member]	Sale of Sirius Group [Member] Common shares and paid-in surplus	Sale of Sirius Group [Member] Retained earnings	Sale of Sirius Group [Member] AOCL, after-tax	Sale of Sirius Group [Member] Parent	Tranzact Non-controlling Interests	Tranzact Total Equity includes Noncontrolling Interest [Member]
Beginning balance at Dec. 31, 2013		$ 1,051.1	$ 2,801.9	$ 52.1	$ 3,905.1	$ 491.7	$ 4,396.8
Increase (Decrease) in Shareholders' Equity
Net income	$ 290.0	0.0	312.2	0.0	312.2	(22.2)	290.0
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	75.3	0.0	0.0	75.3	75.3	0.0	75.3
Other comprehensive income (loss), after-tax														$ 0.0	$ 0.0	$ (8.7)	$ (8.7)	$ (3.0)	$ (11.7)
OCI, Change in foreign currency transaction and translation adjustment, net of tax	(10.7)							$ 0.0	$ 0.0	$ (168.2)	$ (168.2)	$ (0.3)	$ (168.5)																											$ (169.5)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	0.0
Comprehensive income	207.3	0.0	312.2	(101.6)	210.6	(25.5)	185.1
Dividends declared on common shares		0.0	(6.2)	0.0	(6.2)	0.0	(6.2)
Dividends/distributions to non-controlling interests		0.0	0.0	0.0	0.0	(40.7)	(40.7)
Issuances of common shares		4.8	0.0	0.0	4.8	0.0	4.8
Repurchases and retirements of common shares	(134.5)	(37.1)	(97.4)	0.0	(134.5)	0.0	(134.5)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0.0	0.0	0.0	0.0	8.3	8.3
Noncontrolling Interest, Increase from Business Combination							122.1																												$ 122.1
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests							(14.0)																					$ (14.0)
Amortization of restricted share and option awards		15.9	0.0	0.0	15.9	0.8	16.7
Ending balance at Dec. 31, 2014		1,034.7	3,010.5	(49.5)	3,995.7	542.7	4,538.4
Increase (Decrease) in Shareholders' Equity
Net income	279.5	0.0	297.6	0.0	297.6	(18.1)	279.5
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	(34.9)	0.0	0.0	(34.9)	(34.9)	0.0	(34.9)
Other comprehensive income (loss), after-tax														$ 0.0	$ 0.0	0.3	0.3	0.0	0.3
OCI, Change in foreign currency transaction and translation adjustment, net of tax	(0.5)							0.0	0.0	(65.8)	(65.8)	0.0	(65.8)																											(65.0)
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax	0.0
Comprehensive income	197.2	0.0	297.6	(100.4)	197.2	(18.1)	179.1
Dividends declared on common shares		0.0	(6.0)	0.0	(6.0)	0.0	(6.0)
Dividends/distributions to non-controlling interests		0.0	0.0	0.0	0.0	(51.1)	(51.1)
Issuances of common shares		0.9	0.0	0.0	0.9	0.0	0.9
Repurchases and retirements of common shares	(284.2)	(67.0)	(217.2)	0.0	(284.2)	0.0	(284.2)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance		0.0	0.0	0.0	0.0	17.5	17.5
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(5.3)	0.0	0.0	(5.3)	(2.7)	(8.0)																	$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (31.5)	$ (31.5)
Amortization of restricted share and option awards		14.9	0.0	0.0	14.9	(2.0)	12.9
Ending balance at Dec. 31, 2015	3,913.2	978.2	3,084.9	(149.9)	3,913.2	454.8	4,368.0
Increase (Decrease) in Shareholders' Equity
Net income	419.8	0.0	412.5	0.0	412.5	7.3	419.8
Net change in unrealized investment (losses) gains on investments in unconsolidated affiliates, after tax	0.0
Other comprehensive income (loss), after-tax																$ 0.6	$ 0.6	$ 0.3	$ 0.9
OCI, Change in foreign currency transaction and translation adjustment, net of tax	0.3							$ 0.0	$ 0.0	$ 31.4	$ 31.4	$ 0.0	$ 31.4																											32.0
Recognition of foreign currency translation and other from sale of Sirius Group, net of tax																				$ 0.0	$ 0.0	$ 0.0	$ 113.3															$ 113.3	$ 113.3	$ 113.3
Comprehensive income	558.1	0.0	412.5	145.3	557.8	7.6	565.4
Dividends declared on common shares		0.0	(5.4)	0.0	(5.4)	0.0	(5.4)
Dividends/distributions to non-controlling interests		0.0	0.0	0.0	0.0	(22.7)	(22.7)
Issuances of common shares		9.1	0.0	0.0	9.1	0.0	9.1
Repurchases and retirements of common shares	(887.2)	(192.4)	(694.8)	0.0	(887.2)	0.0	(887.2)
Noncontrolling Interest, Decrease from Deconsolidation																																				$ (250.0)	$ (250.0)				$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ (78.4)	$ (78.4)
Noncontrolling Interest, Increase from Subsidiary Equity Issuance						27.3	27.3
Noncontrolling Interest, Increase from Business Combination							3.3																							$ 0.0	$ 0.0	$ 0.0	$ 0.0	$ 3.3
Noncontrolling Interest, Decrease from Redemptions or Purchase of Interests		(2.7)	0.0	0.0	(2.7)	(8.8)	(11.5)
Amortization of restricted share and option awards		18.5	0.0	0.0	18.5	0.8	19.3
Ending balance at Dec. 31, 2016	$ 3,603.3	$ 810.7	$ 2,797.2	$ (4.6)	$ 3,603.3	$ 133.9	$ 3,737.2