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Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands | 6 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Jun. 30, 2021 | Mar. 31, 2021 | Dec. 31, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | $ 18,073,779 |  |  | $ 18,717,825 |  |  |  |
Estimated Fair Value |  | 2,248,613 |  |  | 1,924,922 |  |  |  |
Estimated Fair Value |  | 20,478,448 |  |  | 20,921,530 |  |  |  |
Gross Unrealized Gains |  | 390,212 |  |  | 657,620 |  |  |  |
Gross Unrealized Losses |  | (78,417) |  |  | (79,654) |  |  |  |
Allowance for expected credit losses |  | (2,124) |  | $ (3,830) | (2,397) |  | $ (5,869) | $ (9,938) | $ 0
Amortized cost |  | 17,764,783 |  |  | 18,143,305 |  |  |  |
Cost |  | 2,248,615 |  |  | 1,924,292 |  |  |  |
Cost or Amortized Cost |  | $ 20,168,777 |  |  | 20,345,961 |  |  |  |
Accounting Standards Update |  | us-gaap:AccountingStandardsUpdate201613Member |  |  |  |  |  |  |
Fixed maturities |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value | [1] | $ 18,229,835 |  |  | 18,996,608 |  |  |  |
Gross Unrealized Gains | [1] | 388,808 |  |  | 654,927 |  |  |  |
Gross Unrealized Losses | [1] | (77,011) |  |  | (77,591) |  |  |  |
Allowance for expected credit losses | [2] | (2,124) | [1] |  | (2,397) |  |  |  |
Amortized cost | [1] | 17,920,162 |  |  | 18,421,669 |  |  |  |
Fixed maturities | Corporate bonds |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 7,129,768 |  |  | 7,856,571 |  |  |  |
Gross Unrealized Gains | [1] | 217,061 |  |  | 414,247 |  |  |  |
Gross Unrealized Losses | [1] | (34,751) |  |  | (34,388) |  |  |  |
Allowance for expected credit losses | [2] | (1,232) | [1] |  | (896) |  |  |  |
Amortized cost | [1] | 6,948,690 |  |  | 7,477,608 |  |  |  |
Fixed maturities | Mortgage backed securities |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 389,041 |  |  | 630,001 |  |  |  |
Gross Unrealized Gains | [1] | 4,480 |  |  | 8,939 |  |  |  |
Gross Unrealized Losses | [1] | (3,479) |  |  | (5,028) |  |  |  |
Allowance for expected credit losses | [2] | (269) | [1] |  | (278) |  |  |  |
Amortized cost | [1] | 388,309 |  |  | 626,368 |  |  |  |
Fixed maturities | Municipal bonds |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 415,483 |  |  | 494,522 |  |  |  |
Gross Unrealized Gains | [1] | 23,153 |  |  | 27,291 |  |  |  |
Gross Unrealized Losses | [1] | (1,660) |  |  | (3,835) |  |  |  |
Allowance for expected credit losses | [2] | (6) | [1] |  | (11) |  |  |  |
Amortized cost | [1] | 393,996 |  |  | 471,077 |  |  |  |
Fixed maturities | Commercial mortgage backed securities |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 266,733 |  |  | 389,900 |  |  |  |
Gross Unrealized Gains | [1] | 4,017 |  |  | 8,722 |  |  |  |
Gross Unrealized Losses | [1] | (611) |  |  | (2,954) |  |  |  |
Allowance for expected credit losses | [2] | (3) | [1] |  | (122) |  |  |  |
Amortized cost | [1] | 263,330 |  |  | 384,254 |  |  |  |
Fixed maturities | US government and government agencies |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 5,091,183 |  |  | 5,557,077 |  |  |  |
Gross Unrealized Gains | [1] | 14,814 |  |  | 22,612 |  |  |  |
Gross Unrealized Losses | [1] | (14,013) |  |  | (12,611) |  |  |  |
Allowance for expected credit losses | [2] | 0 | [1] |  | 0 |  |  |  |
Amortized cost | [1] | 5,090,382 |  |  | 5,547,076 |  |  |  |
Fixed maturities | Non-US government securities |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 2,449,782 |  |  | 2,433,733 |  |  |  |
Gross Unrealized Gains | [1] | 108,823 |  |  | 153,891 |  |  |  |
Gross Unrealized Losses | [1] | (18,441) |  |  | (8,060) |  |  |  |
Allowance for expected credit losses | [2] | (128) | [1] |  | 0 |  |  |  |
Amortized cost | [1] | 2,359,528 |  |  | 2,287,902 |  |  |  |
Fixed maturities | Asset backed securities |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 2,487,845 |  |  | 1,634,804 |  |  |  |
Gross Unrealized Gains | [1] | 16,460 |  |  | 19,225 |  |  |  |
Gross Unrealized Losses | [1] | (4,056) |  |  | (10,715) |  |  |  |
Allowance for expected credit losses | [2] | (486) | [1] |  | (1,090) |  |  |  |
Amortized cost | [1] | 2,475,927 |  |  | 1,627,384 |  |  |  |
Short-term investments |  |  |  |  |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |  |  |  |  |
Estimated Fair Value |  | 2,248,613 |  |  | 1,924,922 |  |  |  |
Gross Unrealized Gains |  | 1,404 |  |  | 2,693 |  |  |  |
Gross Unrealized Losses |  | (1,406) |  |  | (2,063) |  |  |  |
Allowance for expected credit losses |  | 0 | [2] | $ 0 | 0 | [2] | $ 0 | $ (29) | $ 0
Cost |  | $ 2,248,615 |  |  | $ 1,924,292 |  |  |  |
[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.”
[2] | Effective January 1, 2020, the Company adopted ASU 2016-13 and as a result any credit impairment losses on the Company’s available-for-sale investments are recorded as an allowance, subject to reversal.

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