Unpaid losses and loss expenses (A&E Loss Roll-forward) (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015		Dec. 31, 2014
Unpaid Losses and Loss Expenses [Roll Forward]
Balance (gross) at December 31, 2014	$ 1,699
Balance (net) at December 31, 2014	1,041
Incurred activity, gross	243
Incurred activity, net	162	[1]	$ 210
Paid activity, gross	(392)
Paid activity, net	(223)
Balance (gross) at December 31, 2015	1,550		1,699
Balance (net) at December 31, 2015	980		1,041
Asbestos
Unpaid Losses and Loss Expenses [Roll Forward]
Balance (gross) at December 31, 2014	1,500
Balance (net) at December 31, 2014	892
Incurred activity, gross	125
Incurred activity, net	76
Paid activity, gross	(274)
Paid activity, net	(137)
Balance (gross) at December 31, 2015	1,351		1,500
Balance (net) at December 31, 2015	831		892
Environmental
Unpaid Losses and Loss Expenses [Roll Forward]
Balance (gross) at December 31, 2014	199
Balance (net) at December 31, 2014	149
Incurred activity, gross	118
Incurred activity, net	86
Paid activity, gross	(118)
Paid activity, net	(86)
Balance (gross) at December 31, 2015	199		199
Balance (net) at December 31, 2015	$ 149		$ 149

[1] (1)Excludes unallocated loss expenses and the net activity reflects third-party reinsurance other than the aggregate excess of loss reinsurance provided by National Indemnity Company (NICO) to Westchester Specialty (see Westchester Specialty section below).