Fair Value Fair Value - Measurement on a recurring basis (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
US Government Agencies Debt Securities [Member]
Available-for-sale Securities, Fair Value	$ 1,744	$ 1,688
US States and Political Subdivisions Debt Securities [Member]
Available-for-sale Securities, Fair Value	2,317	2,399
Debt Security, Corporate, US [Member]
Available-for-sale Securities, Fair Value	7,064	6,489
Debt Security, Corporate, Non-US [Member]
Available-for-sale Securities, Fair Value	2,353	2,383
Residential Mortgage Backed Securities [Member]
Available-for-sale Securities, Fair Value	595	577
Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities, Fair Value	637	605
Other Asset-Backed Securities [Member]
Available-for-sale Securities, Fair Value	1,149	1,085
Redeemable Preferred Stock [Member]
Available-for-sale Securities, Fair Value	38	29
Debt Securities [Member]
Available-for-sale Securities, Fair Value	15,897	15,255
Recurring
Assets, Fair Value Disclosure	15,977	15,328
Recurring | Level 1
Assets, Fair Value Disclosure	1,674	1,616
Recurring | Level 2
Assets, Fair Value Disclosure	14,160	13,581
Recurring | Level 3
Assets, Fair Value Disclosure	143	131
Recurring | US Government Agencies Debt Securities [Member]
Available-for-sale Securities, Fair Value	1,744	1,688
Recurring | US Government Agencies Debt Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	1,665	1,597
Recurring | US Government Agencies Debt Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	79	91
Recurring | US Government Agencies Debt Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	0	0
Recurring | US States and Political Subdivisions Debt Securities [Member]
Available-for-sale Securities, Fair Value	2,317	2,399
Recurring | US States and Political Subdivisions Debt Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | US States and Political Subdivisions Debt Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	2,317	2,399
Recurring | US States and Political Subdivisions Debt Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	0	0
Recurring | Debt Security, Corporate, US [Member]
Available-for-sale Securities, Fair Value	7,064	6,489
Recurring | Debt Security, Corporate, US [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | Debt Security, Corporate, US [Member] | Level 2
Available-for-sale Securities, Fair Value	7,006	6,422
Recurring | Debt Security, Corporate, US [Member] | Level 3
Available-for-sale Securities, Fair Value	58	67
Recurring | Debt Security, Corporate, Non-US [Member]
Available-for-sale Securities, Fair Value	2,353	2,383
Recurring | Debt Security, Corporate, Non-US [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | Debt Security, Corporate, Non-US [Member] | Level 2
Available-for-sale Securities, Fair Value	2,350	2,380
Recurring | Debt Security, Corporate, Non-US [Member] | Level 3
Available-for-sale Securities, Fair Value	3	3
Recurring | Residential Mortgage Backed Securities [Member]
Available-for-sale Securities, Fair Value	595	577
Recurring | Residential Mortgage Backed Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | Residential Mortgage Backed Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	595	577
Recurring | Residential Mortgage Backed Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	0	0
Recurring | Commercial Mortgage Backed Securities [Member]
Available-for-sale Securities, Fair Value	637	605
Recurring | Commercial Mortgage Backed Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | Commercial Mortgage Backed Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	637	605
Recurring | Commercial Mortgage Backed Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	0	0
Recurring | Other Asset-Backed Securities [Member]
Available-for-sale Securities, Fair Value	1,149	1,085
Recurring | Other Asset-Backed Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | Other Asset-Backed Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	1,149	1,085
Recurring | Other Asset-Backed Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	0	0
Recurring | Redeemable Preferred Stock [Member]
Available-for-sale Securities, Fair Value	38	29
Recurring | Redeemable Preferred Stock [Member] | Level 1
Available-for-sale Securities, Fair Value	0	0
Recurring | Redeemable Preferred Stock [Member] | Level 2
Available-for-sale Securities, Fair Value	27	22
Recurring | Redeemable Preferred Stock [Member] | Level 3
Available-for-sale Securities, Fair Value	11	7
Recurring | Debt Securities [Member]
Available-for-sale Securities, Fair Value	15,897	15,255
Recurring | Debt Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	1,665	1,597
Recurring | Debt Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	14,160	13,581
Recurring | Debt Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	72	77
Recurring | Equity Securities [Member]
Available-for-sale Securities, Fair Value	80	73
Recurring | Equity Securities [Member] | Level 1
Available-for-sale Securities, Fair Value	9	19
Recurring | Equity Securities [Member] | Level 2
Available-for-sale Securities, Fair Value	0	0
Recurring | Equity Securities [Member] | Level 3
Available-for-sale Securities, Fair Value	$ 71	$ 54