Investments (Allowance for Credit Losses and Recorded Investment) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for credit losses:
Individually evaluated for impairment	$ 5	$ 6
Collectively evaluated for impairment	102	100
Total ending balance	107	106	$ 112
Recorded Investment:
Individually evaluated for impairment	149	148
Collectively evaluated for impairment	54,873	52,737
Total ending balance	55,022	52,885
Commercial Mortgage Loans
Allowance for credit losses:
Individually evaluated for impairment	5	6
Collectively evaluated for impairment	93	90
Total ending balance	98	96	97
Recorded Investment:
Individually evaluated for impairment	80	116
Collectively evaluated for impairment	50,893	48,911
Total ending balance	50,973	49,027
Agricultural Property Loans
Allowance for credit losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2	2
Total ending balance	2	2	2
Recorded Investment:
Individually evaluated for impairment	67	30
Collectively evaluated for impairment	3,123	2,928
Total ending balance	3,190	2,958
Residential Property Loans
Allowance for credit losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	2	2
Total ending balance	2	2	3
Recorded Investment:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	228	252
Total ending balance	228	252
Other Collateralized Loans
Allowance for credit losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	0	0
Total ending balance	0	0	0
Recorded Investment:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	7	10
Total ending balance	7	10
Uncollateralized Loans
Allowance for credit losses:
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	5	6
Total ending balance	5	6	$ 10
Recorded Investment:
Individually evaluated for impairment	2	2
Collectively evaluated for impairment	622	636
Total ending balance	$ 624	$ 638