Investments (Valuation Allowance Rollforward by Portfolio Segment) (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Loan and Lease Losses, Real Estate	$ 336	$ 316	$ 314	$ 304
Provision for Loan and Lease Losses	(28)	(25)
Allowance for Loan and Lease Losses, Write-offs	6	13
Agricultural Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Loan and Lease Losses, Real Estate	42	41	41	39
Provision for Loan and Lease Losses	(1)	(4)
Allowance for Loan and Lease Losses, Write-offs	0	2
Residential Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Loan and Lease Losses, Real Estate	62	62	59	63
Provision for Loan and Lease Losses	(9)	(10)
Allowance for Loan and Lease Losses, Write-offs	6	11
Commercial Portfolio Segment [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Allowance for Loan and Lease Losses, Real Estate	232	213	$ 214	$ 202
Provision for Loan and Lease Losses	(18)	(11)
Allowance for Loan and Lease Losses, Write-offs	$ 0	$ 0

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No definition available.

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The reserve to cover probable credit losses related to specifically identified fixed maturity real estate loans as well as probable credit losses inherent in the remainder of that loan portfolio.

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-Publisher FASB
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Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

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Amount of expense related to estimated loss from loan and lease transactions.

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