Claim and Claim Adjustment Expense Reserves (Specialty - Surety - 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
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016	Dec. 31, 2016
Claims Development [Line Items]
Net prior year claim and allocated claim adjustment expense reserve development, Surety	$ (84)	$ (63)	$ (69)
Surety
Claims Development [Line Items]
Cumulative Number of Claims	(82)	(65)	(65)
Total net development for accident years prior to 2008	1	2	(4)
Total unallocated claim adjustment expense development	(3)	0	0
Net prior year claim and allocated claim adjustment expense reserve development, Surety	(84)	(63)	(69)
Surety | 2008
Claims Development [Line Items]
Cumulative Number of Claims	0	(1)	(3)	$ (4)	$ (9)	$ (7)	$ (5)	$ (41)	$ 0								$ (70)
Surety | 2009
Claims Development [Line Items]
Cumulative Number of Claims	0	1	(7)	(9)	(17)	(18)	(11)	$ 0								$ (61)
Surety | 2010
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(3)	(10)	(8)	(27)	(1)	$ 0								$ (53)
Surety | 2011
Claims Development [Line Items]
Cumulative Number of Claims	(4)	(5)	(12)	(29)	(5)	$ 1								$ (54)
Surety | 2012
Claims Development [Line Items]
Cumulative Number of Claims	(7)	(18)	(28)	(24)	$ 2								$ (75)
Surety | 2013
Claims Development [Line Items]
Cumulative Number of Claims	(15)	(9)	(6)	$ 1								$ (29)
Surety | 2014
Claims Development [Line Items]
Cumulative Number of Claims	(25)	(30)	$ 1								$ (54)
Surety | 2015
Claims Development [Line Items]
Cumulative Number of Claims	(27)	0								$ (27)
Surety | 2016
Claims Development [Line Items]
Cumulative Number of Claims	$ 0	$ 0