Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Specialty - Surety (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009	Dec. 31, 2008	Dec. 31, 2007
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	$ 4,167	$ 4,089	$ 4,355
Liability for Unpaid Claims and Claims Adjustment Expense, Net	18,249	18,576	18,927	$ 19,117
Specialty - Surety [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	437
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	350
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	16
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	28
Liability for Unpaid Claims and Claims Adjustment Expense, Net	394
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2007 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	50	49	48	46	$ 46	$ 46	$ 45	$ 40	$ 30	$ 12
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2008 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	43	43	43	43	42	39	35	27	$ 9
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2009 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	47	46	45	43	41	34	24	$ 13
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2010 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	55	58	57	55	50	34	$ 13
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2011 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	60	60	58	55	42	$ 19
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2012 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	35	35	34	32	$ 5
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2013 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	78	69	40	$ 16
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2014 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	38	30	$ 7
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2015 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	26	$ 7
Specialty - Surety [Member] | Short-duration Insurance Contracts, Accident Year 2016 [Member]
Short-Duration Insurance Contracts, Net Cumulative Paid Claim and Claim Adjustment Expenses [Line Items]
Short-duration Insurance Contracts, Cumulative Paid Claims and Allocated Claim Adjustment Expense, Net	$ 5