Claim and Claim Adjustment Expense Reserves (Specialty - Other Professional Liability and Management Liability - Net Cumulative Paid Claim and Claim Adjustment Expenses) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2019	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
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	$ 4,584	$ 4,285	$ 4,366
Total net liability for unpaid claim and claim adjustment expenses	17,885	17,965	18,070	$ 18,249
Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Total net liability for unpaid claim and claim adjustment expenses	4,676	4,831
Other Professional Liability and Management Liability | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	5,865
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	2,601
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2010	88
Liability for unallocated claim adjustment expenses for accident years presented	50
Total net liability for unpaid claim and claim adjustment expenses	2,739
Other Professional Liability and Management Liability | 2010 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	790	784	752	721	$ 670	$ 630	$ 541	$ 405	$ 204	$ 31
Other Professional Liability and Management Liability | 2011 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	828	796	781	726	683	605	503	314	$ 71
Other Professional Liability and Management Liability | 2012 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	792	755	711	651	573	400	248	$ 56
Other Professional Liability and Management Liability | 2013 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	771	754	702	618	447	249	$ 54
Other Professional Liability and Management Liability | 2014 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	707	647	515	392	223	$ 51
Other Professional Liability and Management Liability | 2015 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	612	542	404	234	$ 60
Other Professional Liability and Management Liability | 2016 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	625	466	248	$ 64
Other Professional Liability and Management Liability | 2017 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	394	222	$ 57
Other Professional Liability and Management Liability | 2018 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	282	$ 54
Other Professional Liability and Management Liability | 2019 | Specialty
Short Duration Contracts Net Cumulative Paid Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 64