Investments - Scheduled maturities of available for sale fixed maturities (Details) - Fixed maturities $ in Millions	Mar. 31, 2017 USD ($)
Amortized Cost Basis, Fiscal Year Maturity [Abstract]
One year or less	$ 1,038
After one year through five years	6,034
After five years through ten years	12,717
After ten years	4,465
Fixed maturities amortized cost, Subtotal	24,254
ABS (average life of approximately 5 years)	6,548
MBS (average life of approximately 4-1/2 years)	4,643
Amortized Cost	35,445
Fair Value, Fiscal Year Maturity [Abstract]
One year or less	1,051
After one year through five years	6,300
After five years through ten years	12,990
After ten years	4,554
Fixed maturities fair value, Subtotal	24,895
ABS (average life of approximately 5 years)	6,592
MBS (average life of approximately 4-1/2 years)	4,969
Fair Value	$ 36,456
Fair Value Percent, Fiscal Year Maturity [Abstract]
One year or less	3.00%
After one year through five years	17.00%
After five years through ten years	36.00%
After ten years	12.00%
Fixed maturities fair value, Subtotal, Percent	68.00%
ABS (average life of approximately 5 years)	18.00%
MBS (average life of approximately 4-1/2 years)	14.00%
Fair value, Total, Percent	100.00%