Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Incurred Claims and Allocated Claim Adjustment Expenses) (Details) - Surety - Specialty $ in Millions	Dec. 31, 2018 USD ($) claims	Dec. 31, 2017 USD ($)	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 ($)	Dec. 31, 2009 USD ($)
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 778
IBNR	305
2009
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	52	$ 53	$ 53	$ 52	$ 59	$ 68	$ 85	$ 103	$ 114	$ 114
IBNR	$ 1
Cumulative Number of Claims | claims	6,688
2010
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 61	59	63	66	76	84	111	112	$ 112
IBNR	$ 1
Cumulative Number of Claims | claims	5,971
2011
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 62	66	70	75	87	116	121	$ 120
IBNR	$ 1
Cumulative Number of Claims | claims	5,808
2012
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 39	45	52	70	98	122	$ 120
IBNR	$ 2
Cumulative Number of Claims | claims	5,559
2013
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 87	91	106	115	121	$ 120
IBNR	$ 6
Cumulative Number of Claims | claims	5,039
2014
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 60	69	94	124	$ 123
IBNR	$ 19
Cumulative Number of Claims | claims	5,036
2015
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 79	104	131	$ 131
IBNR	$ 36
Cumulative Number of Claims | claims	4,887
2016
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 109	124	$ 124
IBNR	$ 60
Cumulative Number of Claims | claims	5,185
2017
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 115	$ 120
IBNR	$ 78
Cumulative Number of Claims | claims	4,936
2018
Short-duration Insurance Contracts, Net Incurred Claim and Claim Adjustment Expenses [Line Items]
Incurred claims and allocated claim adjustment expense, net	$ 114
IBNR	$ 101
Cumulative Number of Claims | claims	3,105