Claims Development - Excess and Surplus Lines (Details) reported_claim in Thousands, $ in Millions	Dec. 31, 2020 USD ($) reported_claim	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($)	Dec. 31, 2013 USD ($)	Dec. 31, 2012 USD ($)	Dec. 31, 2011 USD ($)
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	$ 3,158
Liabilities for loss and ALAE, net of reinsurance	6,394
Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	836
Cumulative paid losses and ALAE, net of reinsurance	430
All outstanding liabilities before, net of reinsurance	1
Liabilities for loss and ALAE, net of reinsurance	407
2011 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	34	$ 34	$ 34	$ 35	$ 35	$ 36	$ 38	$ 44	$ 47	$ 48
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	1
Cumulative paid losses and ALAE, net of reinsurance	$ 33	33	33	34	32	30	27	23	14	$ 8
2012 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	35	36	35	36	37	40	49	56	67
Total of incurred but not reported liabilities plus expected development on reported losses	$ 0
Cumulative number of reported claims | reported_claim	1
Cumulative paid losses and ALAE, net of reinsurance	$ 34	33	32	31	29	25	19	15	$ 9
2013 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	41	41	41	42	45	54	64	74
Total of incurred but not reported liabilities plus expected development on reported losses	$ 2
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 39	37	34	32	27	20	12	$ 7
2014 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	59	59	60	64	75	82	95
Total of incurred but not reported liabilities plus expected development on reported losses	$ 5
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 51	48	43	37	27	17	$ 9
2015 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	66	65	67	73	81	96
Total of incurred but not reported liabilities plus expected development on reported losses	$ 6
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 54	51	41	29	19	$ 8
2016 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	90	82	84	87	93
Total of incurred but not reported liabilities plus expected development on reported losses	$ 13
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 62	51	39	21	$ 10
2017 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	94	95	95	104
Total of incurred but not reported liabilities plus expected development on reported losses	$ 15
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 57	41	23	$ 11
2018 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	110	109	116
Total of incurred but not reported liabilities plus expected development on reported losses	$ 29
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 50	26	$ 11
2019 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	135	137
Total of incurred but not reported liabilities plus expected development on reported losses	$ 50
Cumulative number of reported claims | reported_claim	3
Cumulative paid losses and ALAE, net of reinsurance	$ 34	$ 13
2020 | Excess and Surplus Lines Insurance
Claims Development
Incurred losses and ALAE, net of reinsurance for the years ended December 31,	172
Total of incurred but not reported liabilities plus expected development on reported losses	$ 105
Cumulative number of reported claims | reported_claim	2
Cumulative paid losses and ALAE, net of reinsurance	$ 16