Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Strengthening or Releasing of Prior Accident Year Reserves) (Details) - Specialty - USD ($) $ in Millions	12 Months Ended									24 Months Ended	36 Months Ended	48 Months Ended	60 Months Ended	72 Months Ended	84 Months Ended	96 Months Ended	108 Months Ended	120 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020	Dec. 31, 2020
Claims Development [Line Items]
Total	$ (15)	$ (69)	$ (127)
Other Professional Liability and Management Liability
Claims Development [Line Items]
Cumulative Number of Claims	4	(38)	(70)
Total net development for accident years prior to 2011	(19)	(17)	(50)
Total unallocated claim adjustment expense development	0	(14)	(7)
Total	(15)	(69)	(127)
Other Professional Liability and Management Liability | 2011
Claims Development [Line Items]
Cumulative Number of Claims	(2)	(3)	(11)	$ (12)	$ (33)	$ (5)	$ 15	$ 26	$ 28									$ 3
Other Professional Liability and Management Liability | 2012
Claims Development [Line Items]
Cumulative Number of Claims	19	(2)	(13)	6	(38)	(9)	(22)	$ (14)									$ (73)
Other Professional Liability and Management Liability | 2013
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(13)	(3)	(19)	(41)	32	$ 10									$ (38)
Other Professional Liability and Management Liability | 2014
Claims Development [Line Items]
Cumulative Number of Claims	(9)	19	4	(54)	(13)	$ 20									$ (33)
Other Professional Liability and Management Liability | 2015
Claims Development [Line Items]
Cumulative Number of Claims	6	(25)	(45)	(15)	$ 4									$ (75)
Other Professional Liability and Management Liability | 2016
Claims Development [Line Items]
Cumulative Number of Claims	3	4	0	$ (1)									$ 6
Other Professional Liability and Management Liability | 2017
Claims Development [Line Items]
Cumulative Number of Claims	(22)	(32)	$ (2)									$ (56)
Other Professional Liability and Management Liability | 2018
Claims Development [Line Items]
Cumulative Number of Claims	5	$ 14									$ 19
Other Professional Liability and Management Liability | 2019
Claims Development [Line Items]
Cumulative Number of Claims	$ 8									$ 8