Claim and Claim Adjustment Expense Reserves (Specialty - Medical Professional Liability - 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
Medical Professional Liability | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	3,082
Net liability for unpaid claim and allocated claim adjustment expenses for the accident years presented	1,416
Net liability for unpaid claim and claim adjustment expenses for accident years prior to 2009	18
Liability for unallocated claim adjustment expenses for accident years presented	23
Total net liability for unpaid claim and claim adjustment expenses	1,457
Medical Professional Liability | 2009 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	300	291	288	280	$ 269	$ 247	$ 211	$ 143	$ 63	$ 9
Medical Professional Liability | 2010 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	350	346	337	326	306	257	173	86	$ 10
Medical Professional Liability | 2011 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	409	398	375	347	295	208	109	$ 17
Medical Professional Liability | 2012 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	457	427	388	323	221	117	$ 14
Medical Professional Liability | 2013 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	462	414	355	255	119	$ 17
Medical Professional Liability | 2014 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	417	359	258	136	$ 23
Medical Professional Liability | 2015 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	313	230	101	$ 22
Medical Professional Liability | 2016 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	246	121	$ 18
Medical Professional Liability | 2017 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	107	$ 19
Medical Professional Liability | 2018 | Specialty
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Cumulative paid claims and allocated claim adjustment expense, net	$ 21