Reinsurance (Components of Written Premiums) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Premiums Written, Net [Abstract]
Direct	$ 12,612	$ 11,894	$ 11,568
Assumed	279	331	360
Ceded	4,832	4,569	4,583
Net	8,059	7,656	7,345
Property and casualty
Premiums Written, Net [Abstract]
Direct	12,168	11,421	11,094
Assumed	229	281	310
Ceded	4,832	4,569	4,583
Net	7,565	7,133	6,821
Long term care
Premiums Written, Net [Abstract]
Direct	444	473	474
Assumed	50	50	50
Ceded	0	0	0
Net	$ 494	$ 523	$ 524
Percentage of assumed premiums written to net premiums written
Premiums Written, Net [Abstract]
Assumed/ Net %	3.50%	4.30%	4.90%
Percentage of assumed premiums written to net premiums written | Property and casualty
Premiums Written, Net [Abstract]
Assumed/ Net %	3.00%	3.90%	4.50%
Percentage of assumed premiums written to net premiums written | Long term care
Premiums Written, Net [Abstract]
Assumed/ Net %	10.10%	9.60%	9.50%