Unpaid losses and loss expenses (Unpaid Losses and Loss Expenses Rollforward) (Details) - USD ($) $ in Millions		12 Months Ended
		Dec. 31, 2017		Dec. 31, 2016		Dec. 31, 2015	Jan. 14, 2016	Apr. 01, 2015
Unpaid Losses and Loss Expenses [Roll Forward]
Gross unpaid losses and loss expenses, beginning of year		$ 60,540		$ 37,303		$ 38,315
Reinsurance recoverable on unpaid losses, beginning of year	[1]	(12,708)	[2]	(10,741)		(11,307)
Net unpaid losses and loss expenses, beginning of year		47,832		26,562		27,008
Acquistion of subsidiaries		0					$ 21,402	$ 417
Total		47,832		47,964		27,425
Net losses and loss expenses incurred in respect of losses incurring in Current Year		19,391		17,256		10,030
PPD, Gross	[3]	(937)		(1,204)		(546)
Total		18,454		16,052		9,484
Net losses and loss expenses incurred in respect of losses paid in Current Year		6,575		5,899		4,053
Net losses and loss expenses incurred in respect of losses paid in Prior Year		10,873		9,816		5,612
Total		17,448		15,715		9,665
Foreign currency revaluation and other		327		(469)		(682)
Net unpaid losses and loss expenses, end of year		49,165		47,832		26,562
Reinsurance recoverable on unpaid losses, end of year	[1]	14,014	[2]	12,708	[2]	10,741
Gross unpaid losses and loss expenses, end of year		63,179		60,540		37,303
prior period development, net adjustments		$ 108		$ 69		$ 0

[1] (1) Net of provision for uncollectible reinsurance.

[2] (1) Net of a provision for uncollectible reinsurance.

[3] (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.