Schedule VI (SUPPLEMENTARY INFORMATION CONCERNING PROPERTY AND CASUALTY OPERATIONS) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Supplemental Information for Property, Casualty Insurance Underwriters [Abstract]
Deferred Policy Acquisition Costs		$ 3,805	$ 3,537	$ 2,219
Net Reserves for Unpaid Losses		49,165	47,832	26,562
Unearned Premiums		15,216	14,779	8,439
Net Premiums Earned		28,054	27,747	16,282
Net Investment Income		2,890	2,656	2,007
Net Losses and Loss Expenses Incurred Related to Current Year		19,391	17,256	10,030
PPD, Gross	[1]	(937)	(1,204)	(546)
Net Losses and Loss Expenses Incurred Related to Prior Year		(829)	(1,135)	(546)
Amortization of Deferred Policy Acquisition Costs		5,519	5,654	2,692
Net Paid Losses and Loss Expenses		17,448	15,715	9,665
Net Premiums Written		$ 28,225	$ 27,074	$ 16,734

[1] (2) Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments, and earned premiums totaling $108 million, $69 million, and nil, for 2017, 2016, and 2015, respectively.