Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions	Dec. 31, 2019 USD ($) claim	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)	Dec. 31, 2010 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 766
IBNR	289
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	61	$ 61	$ 59	$ 63	$ 66	$ 76	$ 84	$ 111	$ 112	$ 112
IBNR	$ 0
Cumulative Number of Claims | claim	5,982
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 62	62	66	70	75	87	116	121	$ 120
IBNR	$ 2
Cumulative Number of Claims | claim	5,813
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 38	39	45	52	70	98	122	$ 120
IBNR	$ 1
Cumulative Number of Claims | claim	5,568
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 83	87	91	106	115	121	$ 120
IBNR	$ 3
Cumulative Number of Claims | claim	5,062
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 45	60	69	94	124	$ 123
IBNR	$ 4
Cumulative Number of Claims | claim	5,078
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 63	79	104	131	$ 131
IBNR	$ 11
Cumulative Number of Claims | claim	4,976
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 84	109	124	$ 124
IBNR	$ 36
Cumulative Number of Claims | claim	5,379
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 103	115	$ 120
IBNR	$ 54
Cumulative Number of Claims | claim	5,496
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 108	$ 114
IBNR	$ 76
Cumulative Number of Claims | claim	5,451
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 119
IBNR	$ 102
Cumulative Number of Claims | claim	3,549