Policyholders' Liabilities (Narrative) (Details)	Dec. 31, 2017
Individual nonparticipating Life Insurance | Domestic | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	1.70%
Individual nonparticipating Life Insurance | Domestic | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	7.80%
Individual nonparticipating Life Insurance | Taiwan | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	6.20%
Individual nonparticipating Life Insurance | Taiwan | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	7.40%
Individual and Group Annuities and Supplementary Contracts
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Percentage of reserves based on interest rates in excess of 8%	0.70%
Individual and Group Annuities and Supplementary Contracts | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	0.00%
Individual and Group Annuities and Supplementary Contracts | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	14.80%
Other | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	2.00%
Liability for Policyholders' Account Balances, interest rate	0.50%
Other | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Future Policy Benefits, interest rate	3.60%
Liability for Policyholders' Account Balances, interest rate	8.00%
Interest-sensitive life contracts | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Policyholders' Account Balances, interest rate	0.80%
Interest-sensitive life contracts | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Policyholders' Account Balances, interest rate	4.40%
Individual annuities | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Policyholders' Account Balances, interest rate	0.00%
Individual annuities | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Policyholders' Account Balances, interest rate	6.30%
Guaranteed interest accounts | Minimum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Policyholders' Account Balances, interest rate	1.00%
Guaranteed interest accounts | Maximum
Liability for Future Policy Benefits and Policyholders' Account Balances, by Product Segment [Line Items]
Liability for Policyholders' Account Balances, interest rate	6.00%