Fair Value - Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2020	Dec. 31, 2019
Assets, Fair Value Disclosure [Abstract]
Debt securities	$ 21,188	$ 16,922
U.S. government securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,469	1,852
States, municipalities and political subdivisions
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,728	2,309
U.S. corporate securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	8,894	7,737
Foreign securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,918	2,348
Residential mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	705	533
Commercial mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,046	700
Other asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,403	1,405
Redeemable preferred securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	25	38
Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	8,079	5,683
Debt securities	21,188	16,922
Equity securities	47	73
Total	29,314	22,678
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	4,210	3,397
Debt securities	2,370	1,785
Equity securities	17	34
Total	6,597	5,216
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	3,869	2,286
Debt securities	18,764	15,088
Equity securities	0	0
Total	22,633	17,374
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Debt securities	54	49
Equity securities	30	39
Total	84	88
Recurring | U.S. government securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,469	1,852
Recurring | U.S. government securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,370	1,785
Recurring | U.S. government securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	99	67
Recurring | U.S. government securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | States, municipalities and political subdivisions
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,728	2,309
Recurring | States, municipalities and political subdivisions | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | States, municipalities and political subdivisions | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,727	2,309
Recurring | States, municipalities and political subdivisions | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	1	0
Recurring | U.S. corporate securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	8,894	7,737
Recurring | U.S. corporate securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | U.S. corporate securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	8,842	7,700
Recurring | U.S. corporate securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	52	37
Recurring | Foreign securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,918	2,348
Recurring | Foreign securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Foreign securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,918	2,348
Recurring | Foreign securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Residential mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	705	533
Recurring | Residential mortgage-backed securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Residential mortgage-backed securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	705	533
Recurring | Residential mortgage-backed securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Commercial mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,046	700
Recurring | Commercial mortgage-backed securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Commercial mortgage-backed securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,046	700
Recurring | Commercial mortgage-backed securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Other asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,403	1,405
Recurring | Other asset-backed securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Other asset-backed securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,403	1,405
Recurring | Other asset-backed securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Redeemable preferred securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	25	38
Recurring | Redeemable preferred securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | Redeemable preferred securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	24	26
Recurring | Redeemable preferred securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	$ 1	$ 12