Fair Values of Financial Instruments (Fair Value by Balance Sheet Grouping) (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
ASSETS
Securities held to maturity	$ 7,466,140	$ 7,337,611
Securities available for sale	3,042,758	3,060,363
Derivatives, index options	149,341	120,644
Fair Values
ASSETS
Securities held to maturity	7,466,140	7,337,611
Securities available for sale	3,042,758	3,060,363
Cash and cash equivalents	109,068	51,247
Mortgage loans	184,561	176,890
Policy loans	104,009	101,092
Other loans	14,991	14,898
Derivatives, index options	149,341	120,644
Short-term investments	0
Life interest in Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	7,370,364	7,367,851
Immediate annuity and supplemental contracts	474,164	470,414
Carrying Values
ASSETS
Securities held to maturity	7,230,187	7,159,259
Securities available for sale	3,042,758	3,060,363
Cash and cash equivalents	109,068	51,247
Mortgage loans	183,698	174,534
Policy loans	57,899	58,699
Other loans	14,541	14,343
Derivatives, index options	149,341	120,644
Short-term investments	0
Life interest in Trust	7,550	7,550
LIABILITIES
Deferred annuity contracts	7,751,899	7,739,337
Immediate annuity and supplemental contracts	443,553	443,226
Level 1
ASSETS
Securities held to maturity	0	0
Securities available for sale	19,618	18,313
Cash and cash equivalents	109,068	51,247
Mortgage loans	0	0
Policy loans	0	0
Other loans	0	0
Derivatives, index options	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 2
ASSETS
Securities held to maturity	7,466,140	7,337,611
Securities available for sale	3,023,140	3,042,050
Cash and cash equivalents	0	0
Mortgage loans	0	0
Policy loans	0	0
Other loans	0	0
Derivatives, index options	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
Securities held to maturity	0	0
Securities available for sale	0	0
Cash and cash equivalents	0	0
Mortgage loans	184,561	176,890
Policy loans	104,009	101,092
Other loans	14,991	14,898
Derivatives, index options	149,341	120,644
Short-term investments	0
Life interest in Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	7,370,364	7,367,851
Immediate annuity and supplemental contracts	$ 474,164	$ 470,414

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