Claim and Claim Adjustment Expense Reserves Short Duration Contracts - Net Cumulative Paid Claim and Claim Adjustment Expenses Specialty - Medical Professional Liability (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 - Medical Professional Liability [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	3,156
Short-duration Insurance Contracts, Liability for Unpaid Claims and Allocated Claim Adjustment Expense, Net	1,716
Liability for Unpaid Claims and Claims Adjustment Expense, Claims Paid, Prior Years	30
Short-duration Insurance Contracts, Liability for Unpaid Claims and Claims Adjustment Expense, Accumulated Unallocated Claim Adjustment Expense	33
Liability for Unpaid Claims and Claims Adjustment Expense, Net	1,779
Short-duration Insurance Contracts, Accident Year 2007 [Member] | Specialty - Medical Professional Liability [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	365	364	352	326	$ 296	$ 247	$ 201	$ 134	$ 68	$ 11
Short-duration Insurance Contracts, Accident Year 2008 [Member] | Specialty - Medical Professional Liability [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	428	409	395	377	332	282	207	90	$ 9
Short-duration Insurance Contracts, Accident Year 2009 [Member] | Specialty - Medical Professional Liability [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	396	377	353	328	278	180	75	$ 9
Short-duration Insurance Contracts, Accident Year 2010 [Member] | Specialty - Medical Professional Liability [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	371	361	338	273	186	93	$ 11
Short-duration Insurance Contracts, Accident Year 2011 [Member] | Specialty - Medical Professional Liability [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	407	379	315	225	121	$ 18
Short-duration Insurance Contracts, Accident Year 2012 [Member] | Specialty - Medical Professional Liability [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	428	359	236	121	$ 15
Short-duration Insurance Contracts, Accident Year 2013 [Member] | Specialty - Medical Professional Liability [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	364	259	121	$ 18
Short-duration Insurance Contracts, Accident Year 2014 [Member] | Specialty - Medical Professional Liability [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	274	149	$ 25
Short-duration Insurance Contracts, Accident Year 2015 [Member] | Specialty - Medical Professional Liability [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	105	$ 22
Short-duration Insurance Contracts, Accident Year 2016 [Member] | Specialty - Medical Professional Liability [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	$ 18