Fair value measurements Fair Value Measurements (Annuitization Experience for GMIB Policies) (Details)	12 Months Ended
	Dec. 31, 2018
Minimum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	0.00%
Maximum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	42.00%
Under 65 years old [Domain]
Annuitization Experience For GMIB Policies [Line Items]
Percentage Of Guaranteed Minimium Income Benefits Guaranteed Value	19.00%
Under 65 years old [Domain] | Minimum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	1.00%
Under 65 years old [Domain] | Maximum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	21.00%
Over 65 years old [Domain]
Annuitization Experience For GMIB Policies [Line Items]
Percentage Of Guaranteed Minimium Income Benefits Guaranteed Value	81.00%
Over 65 years old [Domain] | Minimum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	3.00%
Over 65 years old [Domain] | Maximum
Annuitization Experience For GMIB Policies [Line Items]
Annuitization rate	42.00%