| **Other Comprehensive Income (Details 3) - USD ($) $ in Thousands**                                                                                              | 3 Months Ended | 9 Months Ended |
|------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Other realized investment (gains) losses, net                                                                                                                    | $ (33,374)     | $ (22,914)     | $ (107,193) | $ (57,423) |
| Net investment income                                                                                                                                            | (291,539)      | (363,057)      | (979,080) | (942,082) |
| Interest expense                                                                                                                                                 | (7,648)        | (9,646)        | (26,051) | (28,928) |
| Amortization of unrecognized prior service costs (benefits)                                                                                                      | (150)          | (159)          | (452) | (520) |
| Actuarial (gains)                                                                                                                                                | 2,644          | 3,268          | 7,414 | 9,497 |
| Tax expense or benefit                                                                                                                                           | (3,599)        | (2,813)        | (21,481) | (15,229) |
| Net of tax                                                                                                                                                       | (5,064)        | (3,202)        | (35,369) | (22,447) |
| Amount reclassified from accumulated other comprehensive income (loss)                                                                                           |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Net of tax                                                                                                                                                       | (5,064)        | (3,202)        | (35,369) | (22,447) |
| Unrealized holding (gains) losses, net, arising on fixed maturities, available-for-sale                                                                          |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Net of tax                                                                                                                                                       | (5,503)        | (3,859)        | (35,820) | (24,091) |
| Unrealized holding (gains) losses, net, arising on fixed maturities, available-for-sale | Amount reclassified from accumulated other comprehensive income (loss) |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Other realized investment (gains) losses, net                                                                                                                    | (8,465)        | (5,937)        | (55,107) | (37,063) |
| Total before tax                                                                                                                                                 | (8,465)        | (5,937)        | (55,107) | (37,063) |
| Tax expense or benefit                                                                                                                                           | (2,962)        | (2,078)        | (19,287) | (12,972) |
| Net of tax                                                                                                                                                       | (5,503)        | (3,859)        | (35,820) | (24,091) |
| Unrealized holding (gains) losses, net, arising on cash flow hedges                                                                                              |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Net of tax                                                                                                                                                       | (1,182)        | (1,364)        | (4,074) | (4,191) |
| Unrealized holding (gains) losses, net, arising on cash flow hedges | Amount reclassified from accumulated other comprehensive income (loss)                     |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Net investment income                                                                                                                                            | (2,911)        | (2,099)        | (7,943) | (6,448) |
| Interest expense                                                                                                                                                 | 1,092          | 0              | 1,675 | 0 |
| Total before tax                                                                                                                                                 | (1,819)        | (2,099)        | (6,268) | (6,448) |
| Tax expense or benefit                                                                                                                                           | (637)          | (735)          | (2,194) | (2,257) |
| Net of tax                                                                                                                                                       | (1,182)        | (1,364)        | (4,074) | (4,191) |
| Amortization of employee benefit plan items                                                                                                                      |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Net of tax                                                                                                                                                       | 1,621          | 2,021          | 4,525 | 5,835 |
| Amortization of employee benefit plan items | Amount reclassified from accumulated other comprehensive income (loss)                                             |           |           |   |   |
| **Reclassifications out of accumulated other comprehensive income (loss)**                                                                                       |           |           |   |   |
| Amortization of unrecognized prior service costs (benefits)                                                                                                      | (150)          | (159)          | (452) | (520) |
| Actuarial (gains)                                                                                                                                                | 2,644          | 3,268          | 7,414 | 9,497 |
| Total before tax                                                                                                                                                 | 2,494          | 3,109          | 6,962 | 8,977 |
| Tax expense or benefit                                                                                                                                           | 873            | 1,088          | 2,437 | 3,142 |
| Net of tax                                                                                                                                                       | $ 1,621        | $ 2,021        | $ 4,525 | $ 5,835 |

| X                                                                                                                                                                                                                                                                                                                        |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The net gain (loss) resulting from the sale or disposal of other investments during the period.
+ References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                        |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of gains or losses recognized in net periodic benefit cost.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of the prior service cost or credit recognized in net periodic benefit cost relating to benefit changes attributable to plan participants' prior service pursuant to a plan amendment or a plan initiation.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                    |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of the cost of borrowed funds accounted for as interest expense.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after investment expense, of income earned from investments in securities and real estate. Includes, but is not limited to, real estate investment, policy loans, dividends, and interest. Excludes realized gain (loss) on investments.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                   |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount of tax expense (benefit) of reclassification adjustment from accumulated other comprehensive income for unrealized gain (loss) realized upon the sale of available-for-sale securities.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.
+ References
No definition available.
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|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount before tax of reclassification adjustments of other comprehensive income (loss).
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss).
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