| **Other Comprehensive Income (Details) - USD ($) $ in Thousands**                                                                                                                                                                                                                                                                                           | 3 Months Ended | 9 Months Ended |
|-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|----------------|----------------|
||
| **Other Comprehensive Income [Roll Forward]**                                                                                                                                                                                                                                                                                                               |                |           |   |   |   |
| Beginning balance                                                                                                                                                                                                                                                                                                                                           |                | $ 639,587      | $ 409,371 | $ 233,438 | $ 603,018 |
| Other comprehensive income (loss) before reclassifications                                                                                                                                                                                                                                                                                                  |                | 19,389         | 4,448 | 455,843 | (169,954) |
| Amounts reclassified from AOCI                                                                                                                                                                                                                                                                                                                              |                | (5,064)        | (3,202) | (35,369) | (22,447) |
| Other comprehensive income (loss)                                                                                                                                                                                                                                                                                                                           | [1]            | 14,325         | 1,246 | 420,474 | (192,401) |
| Ending balance                                                                                                                                                                                                                                                                                                                                              |                | 653,912        | 410,617 | 653,912 | 410,617 |
| Unrealized holding gains / losses arising on fixed maturities, available-for- sale                                                                                                                                                                                                                                                                          |                |           |   |   |   |
| **Other Comprehensive Income [Roll Forward]**                                                                                                                                                                                                                                                                                                               |                |           |   |   |   |
| Beginning balance                                                                                                                                                                                                                                                                                                                                           |                | 855,034        | 560,527 | 339,520 | 784,183 |
| Other comprehensive income (loss) before reclassifications                                                                                                                                                                                                                                                                                                  |                | 36,302         | (3,619) | 582,133 | (207,043) |
| Amounts reclassified from AOCI                                                                                                                                                                                                                                                                                                                              |                | (5,503)        | (3,859) | (35,820) | (24,091) |
| Other comprehensive income (loss)                                                                                                                                                                                                                                                                                                                           |                | 30,799         | (7,478) | 546,313 | (231,134) |
| Ending balance                                                                                                                                                                                                                                                                                                                                              |                | 885,833        | 553,049 | 885,833 | 553,049 |
| Unrealized holding gains / losses arising on cash flow hedges                                                                                                                                                                                                                                                                                               |                |           |   |   |   |
| **Other Comprehensive Income [Roll Forward]**                                                                                                                                                                                                                                                                                                               |                |           |   |   |   |
| Beginning balance                                                                                                                                                                                                                                                                                                                                           |                | 45,316         | 30,653 | 45,284 | 33,141 |
| Other comprehensive income (loss) before reclassifications                                                                                                                                                                                                                                                                                                  |                | (5,074)        | 9,623 | (2,150) | 9,962 |
| Amounts reclassified from AOCI                                                                                                                                                                                                                                                                                                                              |                | (1,182)        | (1,364) | (4,074) | (4,191) |
| Other comprehensive income (loss)                                                                                                                                                                                                                                                                                                                           |                | (6,256)        | 8,259 | (6,224) | 5,771 |
| Ending balance                                                                                                                                                                                                                                                                                                                                              |                | 39,060         | 38,912 | 39,060 | 38,912 |
| Future policy benefits, DAC and VOBA adjustments                                                                                                                                                                                                                                                                                                            |                |           |   |   |   |
| **Other Comprehensive Income [Roll Forward]**                                                                                                                                                                                                                                                                                                               |                |           |   |   |   |
| Beginning balance                                                                                                                                                                                                                                                                                                                                           |                | (178,086)      | (79,296) | (65,785) | (108,194) |
| Other comprehensive income (loss) before reclassifications                                                                                                                                                                                                                                                                                                  |                | (11,839)       | (1,549) | (124,140) | 27,349 |
| Amounts reclassified from AOCI                                                                                                                                                                                                                                                                                                                              |                | 0              | 0 | 0 | 0 |
| Other comprehensive income (loss)                                                                                                                                                                                                                                                                                                                           |                | (11,839)       | (1,549) | (124,140) | 27,349 |
| Ending balance                                                                                                                                                                                                                                                                                                                                              |                | (189,925)      | (80,845) | (189,925) | (80,845) |
| Employee benefit plan adjustment                                                                                                                                                                                                                                                                                                                            |                |           |   |   |   |
| **Other Comprehensive Income [Roll Forward]**                                                                                                                                                                                                                                                                                                               |                |           |   |   |   |
| Beginning balance                                                                                                                                                                                                                                                                                                                                           |                | (82,677)       | (102,513) | (85,581) | (106,112) |
| Other comprehensive income (loss) before reclassifications                                                                                                                                                                                                                                                                                                  |                | 0              | (7) | 0 | (222) |
| Amounts reclassified from AOCI                                                                                                                                                                                                                                                                                                                              |                | 1,621          | 2,021 | 4,525 | 5,835 |
| Other comprehensive income (loss)                                                                                                                                                                                                                                                                                                                           |                | 1,621          | 2,014 | 4,525 | 5,613 |
| Ending balance                                                                                                                                                                                                                                                                                                                                              |                | $ (81,056)     | $ (100,499) | $ (81,056) | $ (100,499) |
|                                                                                                                                                                                                                                                                                                                                                             |
| [1]Other comprehensive income (loss) includes the non-credit component of impaired gains (losses), net, on fixed maturities available-for-sale in the amounts of $(959) and $(1,979) for the three months ended September 30, 2016, and 2015, respectively, and $(5,326) and $(3,208) for the nine months ended September 30, 2016, and 2015, respectively. |

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| - Definition
A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.
+ References
No definition available.
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| - Definition
Accumulated change in equity from transactions and other events and circumstances from non-owner sources, net of tax effect, at period end. Excludes Net Income (Loss), and accumulated changes in equity from transactions resulting from investments by owners and distributions to owners. Includes foreign currency translation items, certain pension adjustments, unrealized gains and losses on certain investments in debt and equity securities, other than temporary impairment (OTTI) losses related to factors other than credit losses on available-for-sale and held-to-maturity debt securities that an entity does not intend to sell and it is not more likely than not that the entity will be required to sell before recovery of the amortized cost basis, as well as changes in the fair value of derivatives related to the effective portion of a designated cash flow hedge.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax, before reclassification adjustments of other comprehensive income (loss).
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of other comprehensive income (loss) attributable to parent entity.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount after tax of reclassification adjustments of other comprehensive income (loss).
+ References
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