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Investment Information - Maturity profile of available for sale securities) (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Estimated Fair Value |  |  |
Estimated Fair Value |  | $ 17,207,731 | $ 16,894,526
Amortized Cost |  |  |
Amortized cost |  | 16,733,493 | 16,598,808
Fixed maturities |  |  |
Estimated Fair Value |  |  |
Due in one year or less |  | 344,032 | 428,659
Due after one year through five years |  | 9,873,345 | 10,126,403
Due after five years through 10 years |  | 4,379,828 | 3,317,535
Due after 10 years |  | 428,430 | 411,269
Single maturity date |  | 15,025,635 | 14,283,866
Estimated Fair Value | [1] | 17,665,637 | 17,263,036
Amortized Cost |  |  |
Due in one year or less |  | 339,451 | 423,617
Due after one year through five years |  | 9,645,073 | 9,996,206
Due after five years through 10 years |  | 4,163,392 | 3,219,567
Due after 10 years |  | 396,772 | 388,280
Single maturity date |  | 14,544,688 | 14,027,670
Amortized cost | [1] | 17,187,656 | 16,966,188
Fixed maturities | Mortgage backed securities |  |  |
Estimated Fair Value |  |  |
Securities without single maturity date |  | 610,839 | 562,309
Estimated Fair Value |  | 610,839 | 562,309
Amortized Cost |  |  |
Securities without single maturity date |  | 603,889 | 553,571
Amortized cost | [1] | 603,889 | 553,571
Fixed maturities | Commercial mortgage backed securities |  |  |
Estimated Fair Value |  |  |
Securities without single maturity date |  | 395,682 | 733,108
Estimated Fair Value |  | 395,682 | 733,108
Amortized Cost |  |  |
Securities without single maturity date |  | 396,193 | 720,487
Amortized cost | [1] | 396,193 | 720,487
Fixed maturities | Asset backed securities |  |  |
Estimated Fair Value |  |  |
Securities without single maturity date |  | 1,633,481 | 1,683,753
Estimated Fair Value |  | 1,633,481 | 1,683,753
Amortized Cost |  |  |
Securities without single maturity date |  | 1,642,886 | 1,664,460
Amortized cost | [1] | $ 1,642,886 | $ 1,664,460
[1] | In securities lending transactions, the Company receives collateral in excess of the fair value of the fixed maturities pledged. For purposes of this table, the Company has excluded the collateral received under securities lending, at fair value and included the securities pledged under securities lending, at fair value. See “—Securities Lending Agreements.”

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