Fair value measurements Fair Value Measurements (Annuitization Experience for GMIB Policies) (Details)	12 Months Ended
	Dec. 31, 2017
One Year Of Credible Experience [Member]
Annuitization Experience For GMIB Policies [Line Items]
Percent of GMIB guaranteed value that are represented by clients with several years of annuitization experience	67.00%
Several Years Of Credible Experience [Member]
Annuitization Experience For GMIB Policies [Line Items]
Percent of GMIB guaranteed value that are represented by clients with several years of annuitization experience	3.00%
Several Years Of Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	0.00%
No Credible Experience [Member]
Annuitization Experience For GMIB Policies [Line Items]
Percent of GMIB guaranteed value that are represented by clients with several years of annuitization experience	30.00%
No Credible Experience [Member] | First Year [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	0.00%
No Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	0.00%
Maximum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	100.00%	[1]
Maximum | One Year Of Credible Experience [Member] | First Year [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	52.00%
Maximum | One Year Of Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	100.00%
Maximum | Several Years Of Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	100.00%
Maximum | No Credible Experience [Member] | First Year [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	56.00%
Maximum | No Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	36.00%
Minimum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	0.00%	[1]
Minimum | One Year Of Credible Experience [Member] | First Year [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	2.00%
Minimum | One Year Of Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	1.00%
Minimum | Several Years Of Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	12.00%
Minimum | No Credible Experience [Member] | First Year [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	25.00%
Minimum | No Credible Experience [Member] | Subsequent years [Member]
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	12.00%

[1] Discussion of the most significant inputs used in the fair value measurement of GLB and the sensitivity of those assumptions is included within Note 4 a) Guaranteed living benefits.