Policyholders' Liabilities (Future Policy Benefits) (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Liability for Future Policy Benefit, by Product Segment [Line Items]
Individual and group annuities and supplementary contracts	$ 565,971	$ 548,064
Other contract liabilities	5,482,704	5,071,616
Total future policy benefits	18,593,130	16,503,260
Domestic
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life insurance	11,144,405	9,643,227
Taiwan
Liability for Future Policy Benefit, by Product Segment [Line Items]
Life insurance	$ 1,400,050	$ 1,240,353