Fair Value - Fair Value Measurements (Details) - USD ($) $ in Millions	Dec. 31, 2019	Dec. 31, 2018
Assets, Fair Value Disclosure [Abstract]
Debt securities	$ 16,922	$ 15,255
U.S. government securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,852	1,688
States, municipalities and political subdivisions
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,309	2,399
U.S. corporate securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	7,737	6,489
Foreign securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,348	2,383
Residential mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	533	577
Commercial mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	700	605
Other asset-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,405	1,085
Redeemable preferred securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	38	29
Recurring
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	5,683	4,059
Debt securities	16,922	15,255
Equity securities	73	73
Total	22,678	19,387
Recurring | Level 1
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	3,397	2,619
Debt securities	1,785	1,597
Equity securities	34	19
Total	5,216	4,235
Recurring | Level 2
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	2,286	1,440
Debt securities	15,088	13,581
Equity securities	0	0
Total	17,374	15,021
Recurring | Level 3
Assets, Fair Value Disclosure [Abstract]
Cash and cash equivalents	0	0
Debt securities	49	77
Equity securities	39	54
Total	88	131
Recurring | Brokered Securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities		50
Recurring | U.S. government securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,852	1,688
Recurring | U.S. government securities | Level 1
Assets, Fair Value Disclosure [Abstract]
Debt securities	1,785	1,597
Recurring | U.S. government securities | Level 2
Assets, Fair Value Disclosure [Abstract]
Debt securities	67	91
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	2,309	2,399
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	2,309	2,399
Recurring | States, municipalities and political subdivisions | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	0
Recurring | U.S. corporate securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	7,737	6,489
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	7,700	6,422
Recurring | U.S. corporate securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	37	67
Recurring | Foreign securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	2,348	2,383
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	2,348	2,380
Recurring | Foreign securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	0	3
Recurring | Residential mortgage-backed securities
Assets, Fair Value Disclosure [Abstract]
Debt securities	533	577
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	533	577
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	700	605
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	700	605
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	1,405	1,085
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	1,405	1,085
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	38	29
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	26	22
Recurring | Redeemable preferred securities | Level 3
Assets, Fair Value Disclosure [Abstract]
Debt securities	$ 12	$ 7