Investments (Mortgage Loans by Portfolio Segment) (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017	Mar. 31, 2017	Dec. 31, 2016
Company-held mortgage loans held-for-investment, net
Commercial mortgage loans	$ 46,690	$ 44,375
Percentage of loans receivable on commercial mortgage loans	65.70%	64.60%
Agricultural mortgage loans	$ 13,098	$ 13,014
Percentage of loans receivable on agricultural mortgage loans	18.40%	18.90%
Residential mortgage loans	$ 11,156	$ 11,136
Percentage of loans receivable on residential mortgage loans	15.70%	16.20%
Subtotal	$ 70,944	$ 68,525
Percentage of loans receivable on subtotal	99.80%	99.70%
Valuation allowances	$ (327)	$ (314)	$ (310)	$ (304)
Percentage of loans receivable on valuation allowances	(0.40%)	(0.50%)
Subtotal mortgage loans, net	$ 70,617	$ 68,211
Percentage of loans receivable on subtotal mortgage loans held-for-investment, net	99.40%	99.20%
Percentage of residential mortgage loans - FVO	0.60%	0.80%
Total mortgage loans, net	$ 71,055	$ 68,731
Percentage of loans held for sale on total mortgage loans, net	100.00%	100.00%
Residential — FVO
Company-held mortgage loans held-for-investment, net
Total mortgage loans, net	$ 438	$ 520