Investment Securities Investment Securities (OneBeacon Surplus Notes) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
RBC Score	250.00%
Total realized and unrealized gains (losses)	$ 11.6	$ (226.8)
OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(29.1)	(49.5)
Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	5.1	(15.1)
Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(15.6)	(24.2)
Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(18.6)	(10.2)
Fair Value [Member] | OneBeacon
Surplus notes	71.9	51.5
Par Value [Member] | OneBeacon
Surplus notes	101.0	$ 101.0
Seller Priority Surplus Note [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(6.8)
Seller Priority Surplus Note [Member] | Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	6.2
Seller Priority Surplus Note [Member] | Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(0.2)
Seller Priority Surplus Note [Member] | Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(12.8)
Seller Priority Surplus Note [Member] | Fair Value [Member] | OneBeacon
Surplus notes	51.1
Seller Priority Surplus Note [Member] | Par Value [Member] | OneBeacon
Surplus notes	57.9
Pari Passu Surplus Note [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(22.3)
Pari Passu Surplus Note [Member] | Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(1.1)
Pari Passu Surplus Note [Member] | Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(15.4)
Pari Passu Surplus Note [Member] | Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(5.8)
Pari Passu Surplus Note [Member] | Fair Value [Member] | OneBeacon
Surplus notes	20.8
Pari Passu Surplus Note [Member] | Par Value [Member] | OneBeacon
Surplus notes	$ 43.1