Fair Values of Financial Instruments (Fair Value by Balance Sheet Grouping) (Details) - USD ($) $ in Thousands	Mar. 31, 2016	Dec. 31, 2015	Mar. 31, 2015
ASSETS
Securities held to maturity	$ 7,525,865	$ 7,335,436	$ 7,335,436
Securities available for sale	2,924,376	2,879,583
Derivatives, index options	42,179	38,409
Fair Values [Member]
ASSETS
Securities held to maturity	7,525,864	7,335,436
Securities available for sale	2,924,376	2,879,583
Cash and cash equivalents	141,571	106,007
Mortgage loans	113,436	111,162
Policy loans	112,860	108,550
Other loans	7,895	2,957
Derivatives, index options	42,179	38,409
Life interest in Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	7,265,074	7,288,108
Immediate annuity and supplemental contracts	482,769	461,457
Carrying Values [Member]
ASSETS
Securities held to maturity	7,195,233	7,173,967
Securities available for sale	2,924,376	2,879,583
Cash and cash equivalents	141,571	106,007
Mortgage loans	110,520	108,311
Policy loans	60,488	61,957
Other loans	7,502	2,779
Derivatives, index options	42,179	38,409
Life interest in Trust	7,379	7,379
LIABILITIES
Deferred annuity contracts	7,626,304	7,640,951
Immediate annuity and supplemental contracts	445,075	434,468
Level 1 [Member]
ASSETS
Securities held to maturity	0	0
Securities available for sale	18,136	17,980
Cash and cash equivalents	141,571	106,007
Mortgage loans	0	0
Policy loans	0	0
Other loans	0	0
Derivatives, index options	0	0
Life interest in Trust	0	0
LIABILITIES
Deferred annuity contracts	0	0
Immediate annuity and supplemental contracts	0	0
Level 2 [Member]
ASSETS
Securities held to maturity	7,525,864	7,335,436
Securities available for sale	2,906,240	2,861,603
Cash and cash equivalents	0	0
Mortgage loans	0	0
Policy loans	0	0
Other loans	0	0
Derivatives, index options	0	0
Life interest in Trust	0	$ 0
LIABILITIES
Deferred annuity contracts	0
Immediate annuity and supplemental contracts	0	$ 0
Level 3 [Member]
ASSETS
Securities held to maturity	0	0
Securities available for sale	0	0
Cash and cash equivalents	0	0
Mortgage loans	113,436	111,162
Policy loans	112,860	108,550
Other loans	7,895	2,957
Derivatives, index options	42,179	38,409
Life interest in Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	7,265,074	7,288,108
Immediate annuity and supplemental contracts	$ 482,769	$ 461,457

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