Investment Securities Investment Securities (OneBeacon Surplus Notes) (Details) - USD ($)		12 Months Ended
	Dec. 23, 2014	Dec. 31, 2015	Dec. 31, 2014
Total realized and unrealized gains (losses)		$ 226,800,000	$ (83,300,000)
OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	$ (36,100,000)	(49,500,000)	(35,900,000)
Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(6,800,000)	(15,100,000)	(6,600,000)
Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(12,600,000)	(24,200,000)	(12,600,000)
Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(16,700,000)	(10,200,000)	(16,700,000)
Fair Value [Member] | OneBeacon
Surplus notes	64,900,000	51,500,000	65,100,000
Par Value [Member] | OneBeacon
Surplus notes	101,000,000	101,000,000	$ 101,000,000
Seller Priority Surplus Note [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(14,000,000)	(19,900,000)
Seller Priority Surplus Note [Member] | Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	1,600,000	(400,000)
Seller Priority Surplus Note [Member] | Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(4,600,000)	(11,700,000)
Seller Priority Surplus Note [Member] | Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(11,000,000)	(7,800,000)
Seller Priority Surplus Note [Member] | Fair Value [Member] | OneBeacon
Surplus notes	43,900,000	38,000,000
Seller Priority Surplus Note [Member] | Par Value [Member] | OneBeacon
Surplus notes		57,900,000
Pari Passu Surplus Note [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(22,100,000)	(29,600,000)
Pari Passu Surplus Note [Member] | Current market rates on repayments | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(8,400,000)	(14,700,000)
Pari Passu Surplus Note [Member] | Regulatory Approval [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(8,000,000)	(12,500,000)
Pari Passu Surplus Note [Member] | Liquidity Adjustment [Member] | OneBeacon
Gain (loss) on sale of discontinued operations, net of tax	(5,700,000)	(2,400,000)
Pari Passu Surplus Note [Member] | Fair Value [Member] | OneBeacon
Surplus notes	21,000,000	13,500,000
Pari Passu Surplus Note [Member] | Par Value [Member] | OneBeacon
Surplus notes	$ 43,100,000	43,100,000
Symetra
Total realized and unrealized gains (losses)		$ 258,800,000.0
Surplus Note
RBC Score		250.00%