Investments (Mortgage Loans by Portfolio Segment) (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Company-held mortgage loans held-for-investment, net
Commercial mortgage loans	$ 7,233	$ 6,497
Percentage of loans receivable on commercial mortgage loans	67.90%	69.90%
Agricultural mortgage loans	$ 2,200	$ 1,830
Percentage of loans receivable on agricultural mortgage loans	20.70%	19.70%
Residential mortgage loans	$ 1,138	$ 867
Percentage of loans receivable on residential mortgage loans	10.70%	9.30%
Subtotal (1)	$ 10,571	$ 9,194
Percentage of loans receivable on subtotal	99.30%	98.90%
Valuation allowances (2)	$ (46)	$ (40)
Percentage of loans receivable on valuation allowances	0.40%	0.40%
Subtotal mortgage loans, net	$ 10,525	$ 9,154
Percentage of loans receivable on subtotal mortgage loans held-for-investment, net	98.90%	98.50%
Percentage of loans receivable on commercial mortgage loans held by consolidated securitization entities - fair value option	1.10%	1.50%
Total mortgage loans, net	$ 10,640	$ 9,290
Percentage of total mortgage loans, net	100.00%	100.00%
Consolidated Securitization Entities
Company-held mortgage loans held-for-investment, net
Total mortgage loans, net	$ 115	$ 136