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Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2021 | Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | $ 16,768,363 |  | $ 18,717,825
Estimated Fair Value |  | 3,069,965 |  | 1,924,922
Estimated Fair Value |  | 19,838,328 |  | 20,921,530
Gross Unrealized Gains |  | 255,815 |  | 657,620
Gross Unrealized Losses |  | (115,117) |  | (79,654)
Allowance for expected credit losses |  | (2,111) |  | (2,397)
Amortized cost |  | 16,629,621 |  | 18,143,305
Cost |  | 3,070,120 |  | 1,924,292
Cost or Amortized Cost |  | $ 19,699,741 |  | 20,345,961
Accounting Standards Update |  | Accounting Standards Update 2016-13 [Member] |  |
Fixed maturities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value | [1] | $ 16,768,363 |  | 18,996,608
Gross Unrealized Gains | [1] | 254,190 |  | 654,927
Gross Unrealized Losses | [1] | (113,337) |  | (77,591)
Allowance for expected credit losses | [2] | (2,111) | [1] | (2,397)
Amortized cost | [1] | 16,629,621 |  | 18,421,669
Fixed maturities | Corporate bonds |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 6,403,617 |  | 7,856,571
Gross Unrealized Gains | [1] | 155,246 |  | 414,247
Gross Unrealized Losses | [1] | (40,649) |  | (34,388)
Allowance for expected credit losses | [2] | (1,440) | [1] | (896)
Amortized cost | [1] | 6,290,460 |  | 7,477,608
Fixed maturities | Mortgage backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 405,797 |  | 630,001
Gross Unrealized Gains | [1] | 3,384 |  | 8,939
Gross Unrealized Losses | [1] | (3,966) |  | (5,028)
Allowance for expected credit losses | [2] | (24) | [1] | (278)
Amortized cost | [1] | 406,403 |  | 626,368
Fixed maturities | Municipal bonds |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 382,722 |  | 494,522
Gross Unrealized Gains | [1] | 20,096 |  | 27,291
Gross Unrealized Losses | [1] | (1,206) |  | (3,835)
Allowance for expected credit losses | [2] | (2) | [1] | (11)
Amortized cost | [1] | 363,834 |  | 471,077
Fixed maturities | Commercial mortgage backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 579,424 |  | 389,900
Gross Unrealized Gains | [1] | 3,598 |  | 8,722
Gross Unrealized Losses | [1] | (548) |  | (2,954)
Allowance for expected credit losses | [2] | (3) | [1] | (122)
Amortized cost | [1] | 576,377 |  | 384,254
Fixed maturities | US government and government agencies |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 4,460,515 |  | 5,557,077
Gross Unrealized Gains | [1] | 12,912 |  | 22,612
Gross Unrealized Losses | [1] | (30,320) |  | (12,611)
Allowance for expected credit losses | [2] | 0 | [1] | 0
Amortized cost | [1] | 4,477,923 |  | 5,547,076
Fixed maturities | Non-US government securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 1,863,734 |  | 2,433,733
Gross Unrealized Gains | [1] | 46,378 |  | 153,891
Gross Unrealized Losses | [1] | (28,948) |  | (8,060)
Allowance for expected credit losses | [2] | (82) | [1] | 0
Amortized cost | [1] | 1,846,386 |  | 2,287,902
Fixed maturities | Asset backed securities |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 2,672,554 |  | 1,634,804
Gross Unrealized Gains | [1] | 12,576 |  | 19,225
Gross Unrealized Losses | [1] | (7,700) |  | (10,715)
Allowance for expected credit losses | [2] | (560) | [1] | (1,090)
Amortized cost | [1] | 2,668,238 |  | 1,627,384
Short-term investments |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |
Estimated Fair Value |  | 3,069,965 |  | 1,924,922
Gross Unrealized Gains |  | 1,625 |  | 2,693
Gross Unrealized Losses |  | (1,780) |  | (2,063)
Allowance for expected credit losses | [2] | 0 |  | 0
Cost |  | $ 3,070,120 |  | $ 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.” In September 2021, the Company terminated its securities lending program and no longer enters into securities lending agreements with financial institutions.
[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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