Claim and Claim Adjustment Expense Reserves (Specialty - Surety - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	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
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	$ 4,285	$ 4,366	$ 4,167
Total net liability for unpaid claim and claim adjustment expenses	17,965	18,070	18,249	$ 18,576
Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,831	5,007
Surety | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	436
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	342
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	7
Liability for unallocated claim adjustment expenses for accident years presented	30
Total net liability for unpaid claim and claim adjustment expenses	379
Surety | Specialty | 2009
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	47	47	47	46	$ 45	$ 43	$ 41	$ 34	$ 24	$ 13
Surety | Specialty | 2010
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	52	52	55	58	57	55	50	34	$ 13
Surety | Specialty | 2011
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	57	56	60	60	58	55	42	$ 19
Surety | Specialty | 2012
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	37	36	35	35	34	32	$ 5
Surety | Specialty | 2013
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	78	78	78	69	40	$ 16
Surety | Specialty | 2014
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	38	36	38	30	$ 7
Surety | Specialty | 2015
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	40	38	26	$ 7
Surety | Specialty | 2016
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	45	37	$ 5
Surety | Specialty | 2017
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	37	$ 23
Surety | Specialty | 2018
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 5