Fair Values of Financial Instruments (Fair Value by Balance Sheet Grouping) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
ASSETS
Debt securities held to maturity	$ 7,302,034	$ 7,434,104
Securities available for sale		3,059,609
Fair Value	3,084,789	3,041,131
Derivatives, index options	116,356	194,731
Equity securities	18,173	18,478
Fair Values
ASSETS
Debt securities held to maturity	7,302,034	7,434,104
Securities available for sale		3,059,609
Fair Value	3,084,789
Cash and cash equivalents	82,924	217,624
Mortgage loans	205,533	208,815
Policy loans	94,128	100,230
Other loans	5,688	5,603
Derivatives, index options	116,356	194,731
Equity securities	18,173
Short-term investments	0
Life interest in Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	7,252,309	7,338,637
Immediate annuity and supplemental contracts	430,398	443,437
Carrying Values
ASSETS
Debt securities held to maturity	7,280,200	7,247,024
Securities available for sale		3,059,609
Fair Value	3,084,789
Cash and cash equivalents	82,924	217,624
Mortgage loans	204,948	208,249
Policy loans	55,515	56,405
Other loans	5,533	5,431
Derivatives, index options	116,356	194,731
Equity securities	18,173
Short-term investments	0
Life interest in Trust	8,676	8,676
LIABILITIES
Deferred annuity contracts	7,758,838	7,865,786
Immediate annuity and supplemental contracts	423,056	430,494
Level 1
ASSETS
Debt securities held to maturity	0	0
Securities available for sale		18,478
Fair Value	0	0
Cash and cash equivalents	82,924	217,624
Mortgage loans	0	0
Policy loans	0	0
Other loans	0	0
Derivatives, index options	0	0
Equity securities	18,173	18,478
Short-term investments	0
Life interest in Trust	0	0
LIABILITIES
Deferred annuity contracts	0	0
Immediate annuity and supplemental contracts	0	0
Level 2
ASSETS
Debt securities held to maturity	7,299,770	7,431,810
Securities available for sale		3,041,131
Fair Value	3,084,789	3,041,131
Cash and cash equivalents	0	0
Mortgage loans	0	0
Policy loans	0	0
Other loans	0	0
Derivatives, index options	0	0
Equity securities	0	0
Short-term investments	0
Life interest in Trust	0	0
LIABILITIES
Deferred annuity contracts	0	0
Immediate annuity and supplemental contracts	0	0
Level 3
ASSETS
Debt securities held to maturity	2,264	2,294
Securities available for sale		0
Fair Value	0	0
Cash and cash equivalents	0	0
Mortgage loans	205,533	208,815
Policy loans	94,128	100,230
Other loans	5,688	5,603
Derivatives, index options	116,356	194,731
Equity securities	0	0
Short-term investments	0
Life interest in Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	7,252,309	7,338,637
Immediate annuity and supplemental contracts	$ 430,398	$ 443,437