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Investments (Other Investment Portfolio by Strategy and the Remaining Commitment Amount and Maximum Exposure to Loss Associated With Each Strategy) (Details) - USD ($) $ in Thousands | Jun. 30, 2018 | Dec. 31, 2017
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Carrying Value |  | $ 145,203 | $ 132,268
Other Investments [Member] |  |  |
Carrying Value |  | 145,203 | 132,268
Remaining Commitment |  | 237,178 | 220,999
Maximum Exposure to Loss | [1] | 382,381 | 353,267
Alternative Investments [Member] |  |  |
Carrying Value |  | 124,027 | 115,373
Remaining Commitment |  | 237,178 | 220,999
Maximum Exposure to Loss | [1] | 361,205 | 336,372
Private Equity [Member] | Alternative Investments [Member] |  |  |
Carrying Value |  | 59,681 | 52,251
Remaining Commitment |  | 110,833 | 99,026
Maximum Exposure to Loss | [1] | 170,514 | 151,277
Private Credit [Member] | Alternative Investments [Member] |  |  |
Carrying Value |  | 40,916 | 37,743
Remaining Commitment |  | 89,757 | 94,959
Maximum Exposure to Loss | [1] | 130,673 | 132,702
Real Assets [Member] | Alternative Investments [Member] |  |  |
Carrying Value |  | 23,430 | 25,379
Remaining Commitment |  | 36,588 | 27,014
Maximum Exposure to Loss | [1] | 60,018 | 52,393
Other Securities [Member] |  |  |
Carrying Value |  | 21,176 | 16,895
Remaining Commitment |  | 0 | 0
Maximum Exposure to Loss | [1] | $ 21,176 | $ 16,895
[1] | The maximum exposure to loss includes both the carry value of these investments and the related unfunded commitments. In addition, tax credits that have been previously recognized in Other securities are subject to the risk of recapture, which we do not consider significant.

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