Summary of Significant Accounting Policies Summary of Significant Accounting Policies - LIHC (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Item Effected [Line Items]
Net investment income									$ 86.8	$ 60.8	$ 59.5
Income Tax Expense (Benefit)	$ 23.0	$ 6.7	$ 6.0	$ 9.7	$ 1.0	$ 1.6	$ 2.2	$ (4.6)	$ 45.4	$ 0.2	14.8
OneBeacon Ltd.
Item Effected [Line Items]
Cumulative Effect on Retained Earnings, Net of Tax											0.9	$ 0.4
Accounting Standards Update 2014-01 [Member] | Restatement Adjustment [Member] | OneBeacon Ltd.
Item Effected [Line Items]
Net investment income											1.7	1.9
Income Tax Expense (Benefit)											$ (2.3)	$ (2.2)