Reserves for Future Policy Benefits and Unpaid Losses and Loss Adjustment Expenses Level 4 Prior Accident Years Reserve Development (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	$ 250	$ 228	$ 192
Auto liability [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	54	25	144
Homeowners [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	9	(7)	(6)
Professional Liability [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(36)	(17)	(29)
Package Business [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	28	3	2
Accident and Health Insurance Product Line [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(37)	(7)	(2)
Workers Compensation NY Section 25A [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	0	80
General liability [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	8	(25)	(75)
Fidelity and Surety [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(2)	8	(8)
Commercial property [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(6)	2	(7)
Net asbestos reserves [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	146	212	130
Net environmental reserves [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	55	30	12
Uncollectible reinsurance [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	0	0	(25)
Change in workers compensation discount, including accretion [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	29	30	30
Catastrophes [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	(18)	(45)	(63)
Other reserve re-estimates [Member]
Prior accident years reserve development
Liability for Unpaid Claims and Claims Adjustment Expense, Period Increase (Decrease)	$ 20	$ 19	$ 9