| **Summary of Investments - Ltd partnership Sec lending Special deposits (Details) - USD ($)** | Jun. 30, 2018 | Dec. 31, 2017 |
|-----------------------------------------------------------------------------------------------|---------------|---------------|
| **Nature and activities of the VIEs and the effect on financial statements**                  |          |          |
| Carrying value to loss in relation to the activities of VIEs                                  | $ 63,586,000  | $ 45,540,000  |
| Maximum exposure to loss in relation to the activities of the VIEs                            | 63,586,000    | 45,540,000    |
| **Special deposits and securities lending**                                                   |          |          |
| Amortized cost                                                                                | 98,705,000    |          |
| Securities loaned to third parties at fair value                                              | 93,255,000    | 0             |
| Collateral received                                                                           | 80,013,000    | 0             |
| Collateral fair value                                                                         | 16,502,000    |          |
| Payable under securities lending agreements                                                   | 80,013,000    | $ 0           |
| Corporate debt securities                                                                     |          |          |
| **Special deposits and securities lending**                                                   |          |          |
| Payable under securities lending agreements                                                   | $ 80,013,000  |          |

| X                                                                                                                                                                                                                                                      |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Cash Received For Securities Loaned
+ References
No definition available.
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| X                                                                                                                                                                                                                                                 |
|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Securities Loaned Amortized Cost
+ References
No definition available.
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| X                                                                                                                                                                                                         |
|-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - References
No definition available.
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                |
|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The amount of cash received as security in return for loaning securities to another party.
+ References
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|--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Amount, after the effects of master netting arrangements, of securities loaned to entities in exchange for collateral. Includes liabilities not subject to a master netting arrangement and not elected to be offset.
+ References
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| X                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      |
|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
Fair value of the securities pledged as collateral against securities loaned.
+ References
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|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The carrying amount of the consolidated Variable Interest Entity's assets included in the reporting entity's statement of financial position.
+ References
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|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| - Definition
The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).
+ References
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