Fair Values of Financial Instruments (Fair Value by Balance Sheet Grouping) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Mar. 31, 2012
ASSETS
Securities held to maturity	$ 6,588,874	$ 6,503,627
Securities available for sale	2,904,202	2,826,001
Derivatives, index options	130,967	57,890	130,967	57,890
Carrying Values [Member]
ASSETS
Securities held to maturity	6,078,141	5,962,120
Securities available for sale	2,904,202	2,826,001
Cash and short-term investments	86,816	124,561
Mortgage loans	125,539	142,170
Policy loans	71,143	71,549
Other loans	12,169	14,997
Derivatives, index options	130,967	57,890
Life interest in Libbie Shearn Moody Trust	0	0
LIABILITIES
Deferred annuity contracts	7,020,784	6,907,055
Immediate annuity and supplemental contracts	489,011	492,853
Fair Values [Member]
ASSETS
Securities held to maturity	6,588,874	6,503,627
Securities available for sale	2,904,202	2,826,001
Cash and short-term investments	86,816	124,561
Mortgage loans	132,063	147,365
Policy loans	71,143	71,549
Other loans	12,490	15,273
Derivatives, index options	130,967	57,890
Life interest in Libbie Shearn Moody Trust	12,775	12,775
LIABILITIES
Deferred annuity contracts	6,734,914	6,624,111
Immediate annuity and supplemental contracts	$ 526,909	$ 531,857

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