Related party transaction (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Other investments	$ 4,672				$ 4,519				$ 4,672	$ 4,519
Net premiums written									29,244	28,145	$ 17,713
Losses and loss expenses	4,272	$ 6,247	$ 4,146	$ 3,789	3,855	$ 4,269	$ 4,254	$ 3,674	18,454	16,052	9,484
Insurance and reinsurance balances payable	5,868				5,637				5,868	5,637
Reinsurance recoverable on losses and loss expenses	15,034				13,577				15,034	13,577
Ceded Premiums Written									7,132	6,838	6,098
All Related Party [Member]
Related Party Transaction [Line Items]
Other investments	86								86
Starr Technical Risk Agency and Affiliates [Member]
Related Party Transaction [Line Items]
Paid commissions									101	145	60
Losses and loss expenses									438	313	137
Insurance and reinsurance balances payable	44				72				44	72
Reinsurance recoverable on losses and loss expenses	557				412				557	412
Gross premiums written									464	658	305
Ceded Premiums Written									175	208	78
Minimum Amount of Program Business to be Written to Earn Profit Sharing									20
Ceded Commissions									37	56	$ 19
ABR Reinsurance Capital Holdings Ltd. [Member]
Related Party Transaction [Line Items]
Insurance and reinsurance balances payable	51				53				51	53
Reinsurance recoverable on losses and loss expenses	$ 365				$ 148				365	148
Ceded Premiums Written									342	288
Fees and Commissions, Other									$ 94	$ 66
Equity Method Investment, Ownership Percentage					11.30%					11.30%