Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Specialty Reconciliation (Details) - USD ($) $ in Millions	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 18,249	$ 18,576	$ 18,927	$ 19,117
Specialty - Medical Professional Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,779
Specialty - Other Professional and Management Liability [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	3,063
Specialty - Surety [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	394
Specialty - Warranty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	37
Specialty - Other [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	131
Specialty [Member]
Short-duration Insurance Contracts, Reconciliation of Claims Development to Liability [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Net	$ 5,404	$ 5,568