Accounting Policies - Unpaid Claims and Claims Expenses (Policies)	9 Months Ended
	Sep. 30, 2018
Global Health Benefits Segment [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Liabilities for unpaid claims and claims expenses	Medical costs payable for the Global Health Care segment reflects estimates of the ultimate cost of claims that have been incurred but not reported, including expected development on reported claims, those that have been reported but not yet paid (reported claims in process), and other medical care expenses and services payable that are primarily comprised of accruals for incentives and other amounts payable to health care professionals and facilities. See Note 7 to the Consolidated Financial Statements in the Company’s 2017 Form 10-K for further information about the assumptions and estimates used to establish this liability. Reinsurance and other amounts recoverable in the above table includes amounts due from reinsurers and policyholders to cover incurred but not reported and pending claims for certain business for which the Company administers the plan benefits but the right of offset does not exist. See Note 8 for additional information on reinsurance. Incurred costs related to prior years in the table above, although adjusted through shareholders’ net income, do not directly correspond to an increase or decrease to shareholders’ net income. The primary reason for this difference is that decreases to prior year incurred costs pertaining to the portion of the liability established for moderately adverse conditions are not considered as impacting shareholders’ net income if they are offset by increases in the current year provision for moderately adverse conditions.
Group Disability and Life, Global Supplemental Beneftits and Other Operations [Member]
Liability For Unpaid Claims And Claims Adjustment Expense [Line Items]
Liabilities for unpaid claims and claims expenses	Reinsurance in the table above reflects amounts due from reinsurers related to unpaid claims liabilities. The Company's insurance subsidiaries enter into agreements with other companies primarily to limit losses from large exposures and to permit recovery of a portion of incurred losses. See Note 8 for additional information on reinsurance. The majority of the liability for unpaid claims and claim expenses is related to disability claims with long-tailed payouts. Interest earned on assets backing these liabilities is an integral part of pricing and reserving. Therefore, interest accreted on prior year balances is shown as a separate component of prior year incurred claims. This interest is calculated by applying the average discount rate used in determining the liability balance to the average liability balance over the period. The remaining prior year incurred claims amount primarily reflects updates to the Company’s liability estimates and variances between actual experience during the period relative to the assumptions and expectations reflected in determining the liability. Assumptions reflect the Company’s expectations over the life of the book of business and will vary from actual experience in any period, both favorably and unfavorably, with variation in resolution rates being the most significant driver for the long-term disability business and variations in mortality and morbidity being the most significant factors for other business. GMIB In this business, the Company reinsured contracts with issuers of GMIB products. The Company’s exposure represents the excess of a contractually guaranteed amount over the level of variable annuity account values. Payment by the Company depends on the actual account value in the underlying mutual funds and the level of interest rates when the contractholders elect to receive minimum income payments that must occur within 30 days of a policy anniversary after the appropriate waiting period. The Company has purchased retrocessional coverage (“GMIB assets”) for these contracts.