Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions	Dec. 31, 2020 USD ($) claim	Dec. 31, 2019 USD ($)	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 ($)	Dec. 31, 2011 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 766
IBNR	290
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	62	$ 62	$ 62	$ 66	$ 70	$ 75	$ 87	$ 116	$ 121	$ 120
IBNR	$ 1
Cumulative Number of Claims | claim	5,828
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 37	38	39	45	52	70	98	122	$ 120
IBNR	$ 1
Cumulative Number of Claims | claim	5,577
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 82	83	87	91	106	115	121	$ 120
IBNR	$ 1
Cumulative Number of Claims | claim	5,078
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 45	45	60	69	94	124	$ 123
IBNR	$ 1
Cumulative Number of Claims | claim	5,102
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 58	63	79	104	131	$ 131
IBNR	$ 6
Cumulative Number of Claims | claim	5,026
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 67	84	109	124	$ 124
IBNR	$ 12
Cumulative Number of Claims | claim	5,469
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 84	103	115	$ 120
IBNR	$ 31
Cumulative Number of Claims | claim	5,706
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 91	108	$ 114
IBNR	$ 48
Cumulative Number of Claims | claim	5,920
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 112	$ 119
IBNR	$ 67
Cumulative Number of Claims | claim	5,344
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 128
IBNR	$ 122
Cumulative Number of Claims | claim	2,527