Certain Long-Duration Contracts With Guarantees (Separate Account Investment Options) (Details) - Annuity Contracts - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Separate Account Investment Options	$ 112,844,441	$ 102,262,328
Equity funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Separate Account Investment Options	65,755,874	59,482,605
Bond funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Separate Account Investment Options	44,672,897	36,221,736
Money market funds
Schedule of Fair Value of Separate Accounts by Major Category of Investment [Line Items]
Separate Account Investment Options	$ 2,415,670	$ 6,557,987