Investments (Mortgage Loans by Portfolio Segment) (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Company-held mortgage loans held-for-investment, net
Commercial Mortgage Loans	$ 8,058	$ 7,233
Percentage of loans receivable on commercial mortgage loans	65.80%	67.90%
Agricultural Mortgage Loans	$ 2,557	$ 2,200
Percentage of loans receivable on agricultural mortgage loans	20.90%	20.70%
Residential Mortgage Loans	$ 1,577	$ 1,138
Percentage of loans receivable on residential mortgage loans	12.90%	10.70%
Subtotal (1)	$ 12,192	$ 10,571
Percentage of loans receivable on subtotal	99.60%	99.30%
Valuation allowances (2)	$ (51)	$ (46)
Percentage of loans receivable on valuation allowances	(0.40%)	(0.40%)
Subtotal mortgage loans, net	$ 12,141	$ 10,525
Percentage of loans receivable on subtotal mortgage loans held-for-investment, net	99.20%	98.90%
Mortgage Loans on Real Estate, Commerical and Consumer Net	$ 12,237	$ 10,640
Percentage of loans receivable on commercial mortgage loans held by consolidated securitization entities - fair value option	0.80%	1.10%
Percentage of loans held for sale on total mortgage loans, net	100.00%	100.00%
Variable Interest Entity, Primary Beneficiary [Member]
Company-held mortgage loans held-for-investment, net
Mortgage Loans on Real Estate, Commerical and Consumer Net	$ 96	$ 115