Investment Securities OneBeacon Surplus Notes (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016	Dec. 31, 2016	Dec. 23, 2014
Interest Paid	$ 0.2	$ 0.2
OneBeacon [Member]
Assets, Fair Value Adjustment	(31.4)		$ (29.1)
Seller Priority Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	$ (9.8)
Surplus Note
Interest Paid		$ 2.4
Surplus Note | OneBeacon [Member]
Basis points	4.00%		2.50%
Pari Passu Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	$ (21.6)
Surplus Note
RBC Score	250.00%		250.00%
Par Value [Member] | OneBeacon [Member]
Surplus Notes	$ 101.0		$ 101.0	$ 101.0
Par Value [Member] | Seller Priority Surplus Note [Member] | OneBeacon [Member]
Surplus Notes	57.9
Par Value [Member] | Pari Passu Surplus Note [Member] | OneBeacon [Member]
Surplus Notes	43.1
Fair Value [Member] | OneBeacon [Member]
Surplus Notes	69.6		71.9
Fair Value [Member] | Seller Priority Surplus Note [Member] | OneBeacon [Member]
Surplus Notes	48.1
Fair Value [Member] | Pari Passu Surplus Note [Member] | OneBeacon [Member]
Surplus Notes	21.5
Current market rates on repayments [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	9.0		5.1
Current market rates on repayments [Member] | Seller Priority Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	7.2
Current market rates on repayments [Member] | Pari Passu Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	1.8
Regulatory Approval [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	(10.3)		(15.6)
Regulatory Approval [Member] | Seller Priority Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	2.9
Regulatory Approval [Member] | Pari Passu Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	(13.2)
Liquidity Adjustment [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	(30.1)		$ (18.6)
Liquidity Adjustment [Member] | Seller Priority Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	(19.9)
Liquidity Adjustment [Member] | Pari Passu Surplus Note [Member] | OneBeacon [Member]
Assets, Fair Value Adjustment	$ (10.2)