Fair Value of Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2014		Dec. 31, 2013
U.S. Government and federally sponsored agency obligations:
Carrying Amount	$ 6,573,351		$ 6,313,189
Fair Value	6,573,351		6,313,189
Investments, Carrying Amount	6,801,280		6,539,473
Separate Account (variable annuity) assets	1,745,044	[1]	1,747,995	[1]
Separate Account (variable annuity) assets, Fair Value	1,745,044	[1]	1,747,995	[1]
Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	6,398,442		6,009,573
Fair Value	6,398,442		6,009,573
Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Investments, Carrying Amount	75,484		206,758
Investments, Fair Value	75,484		206,758
Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	577,268		569,725
Fair Value	577,268		569,725
Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	466,990		435,574
Fair Value	466,990		435,574
Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	1,546,159		1,471,527
Fair Value	1,546,159		1,471,527
Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	55,863		54,951
Fair Value	55,863		54,951
Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	2,784,638		2,614,409
Fair Value	2,784,638		2,614,409
Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	967,524		863,387
Fair Value	967,524		863,387
Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Carrying Amount	94,189		91,858
Fair Value	94,189		91,858
Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Investment In Federal Home Loan Bank Stock Carrying Value	5,236		5,000
Investment in Federal Home Loan Bank Stock, Fair Value Disclosure	5,236		5,000
Other Policyholders Funds, embedded derivatives [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Financial Liabilities Other policyholders funds, embedded derivatives, Fair Value	2,747		0
Financial Liabilities Other policyholders funds, embedded derivatives, Carrying Value	2,747		0
Level 1 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	184,681		308,571
Separate Account (variable annuity) assets, Fair Value	1,745,044	[1]	1,747,995	[1]
Level 1 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	33,279		27,938
Level 1 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Investments, Fair Value	75,080		206,354
Level 1 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 1 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	22,857		17,757
Level 1 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 1 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 1 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	10,422		10,181
Level 1 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 1 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	76,322		74,279
Level 1 [Member] | Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Investment in Federal Home Loan Bank Stock, Fair Value Disclosure	0		0
Level 1 [Member] | Other Policyholders Funds, embedded derivatives [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Financial Liabilities Other policyholders funds, embedded derivatives, Fair Value	0		0
Level 2 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	6,263,130		5,895,083
Separate Account (variable annuity) assets, Fair Value	0	[1]	0	[1]
Level 2 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	6,239,629		5,872,106
Level 2 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Investments, Fair Value	404		404
Level 2 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	577,268		569,725
Level 2 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	444,133		417,817
Level 2 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	1,533,380		1,468,833
Level 2 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	55,863		54,951
Level 2 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	2,714,012		2,543,402
Level 2 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	914,973		817,378
Level 2 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	17,861		17,573
Level 2 [Member] | Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Investment in Federal Home Loan Bank Stock, Fair Value Disclosure	5,236		5,000
Level 2 [Member] | Other Policyholders Funds, embedded derivatives [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Financial Liabilities Other policyholders funds, embedded derivatives, Fair Value	0		0
Level 3 [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	125,540		109,535
Separate Account (variable annuity) assets, Fair Value	0	[1]	0	[1]
Level 3 [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	125,534		109,529
Level 3 [Member] | Short-term investments [Member]
U.S. Government and federally sponsored agency obligations:
Investments, Fair Value	0		0
Level 3 [Member] | Mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 3 [Member] | Other, including U.S. Treasury Securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 3 [Member] | Municipal bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	12,779		2,694
Level 3 [Member] | Foreign government bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	0		0
Level 3 [Member] | Corporate bonds [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	60,204		60,826
Level 3 [Member] | Other mortgage-backed securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	52,551		46,009
Level 3 [Member] | Equity securities [Member]
U.S. Government and federally sponsored agency obligations:
Fair Value	6		6
Level 3 [Member] | Other Investment [Member]
U.S. Government and federally sponsored agency obligations:
Investment in Federal Home Loan Bank Stock, Fair Value Disclosure	0		0
Level 3 [Member] | Other Policyholders Funds, embedded derivatives [Member] | Recurring [Member]
U.S. Government and federally sponsored agency obligations:
Financial Liabilities Other policyholders funds, embedded derivatives, Fair Value	$ 2,747		$ 0

[1] Separate Account (variable annuity) liabilities are set equal to Separate Account (variable annuity) assets.