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Consolidated Balance Sheets (Parentheticals) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019
--- | --- | ---
Fixed maturities available for sale, at amortized cost | $ 18,143,305 | $ 16,598,808
Short-term investments available for sale, at amortized cost | 1,924,292 | 957,283
Collateral received under securities lending agreements, at amortized cost | 301,089 | 388,366
Investments accounted for using the fair value option | 3,824,796 | 3,663,477
Securities pledged under securities lending, at amortized cost | 294,493 | 378,738
Allowance for credit losses on premiums receivable | 37,781 | 21,003
Allowance for credit losses on reinsurance recoverable | 11,636 | 1,364
Allowance for credit loss on contractholder receivable | $ 8,638 | $ 0
Common shares, par value per share | $ 0.0011 | $ 0.0011
Common shares issued (shares) | 579,000,841 | 574,617,195
Common shares held in treasury (shares) | 172,280,199 | 168,997,994
Short-term investments |  |
Short-term investments available for sale, at amortized cost | $ 1,924,292 | $ 957,283
Allowance for credit losses on investments | 0 |
Fixed maturities |  |
Allowance for credit losses on investments | $ 2,397 |

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