Claim, Claim Adjustment Expense and Future Policy Benefit Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions	Dec. 31, 2021 USD ($) claim	Dec. 31, 2020 USD ($)	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 ($)	Dec. 31, 2012 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 765
IBNR	328
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	36	$ 37	$ 38	$ 39	$ 45	$ 52	$ 70	$ 98	$ 122	$ 120
IBNR	$ 1
Cumulative Number of Claims | claim	5,586
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 82	82	83	87	91	106	115	121	$ 120
IBNR	$ 2
Cumulative Number of Claims | claim	5,088
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 43	45	45	60	69	94	124	$ 123
IBNR	$ 1
Cumulative Number of Claims | claim	5,118
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 53	58	63	79	104	131	$ 131
IBNR	$ 9
Cumulative Number of Claims | claim	5,055
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 64	67	84	109	124	$ 124
IBNR	$ 10
Cumulative Number of Claims | claim	5,521
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 71	84	103	115	$ 120
IBNR	$ 9
Cumulative Number of Claims | claim	5,795
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 62	91	108	$ 114
IBNR	$ 19
Cumulative Number of Claims | claim	6,097
2019
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 98	112	$ 119
IBNR	$ 44
Cumulative Number of Claims | claim	5,816
2020
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 119	$ 128
IBNR	$ 104
Cumulative Number of Claims | claim	4,006
2021
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 137
IBNR	$ 129
Cumulative Number of Claims | claim	2,592