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Investment Information - Summary of available for sale securities (Details) - USD ($) $ in Thousands | Sep. 30, 2020 | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | $ 18,452,888 |  |  | $ 16,894,526
Estimated Fair Value |  | 2,039,097 |  |  | 956,546
Estimated Fair Value |  | 20,554,734 |  |  | 18,219,582
Gross Unrealized Gains |  | 596,977 |  |  | 351,114
Gross Unrealized Losses |  | (94,731) |  |  | (55,003)
Allowance for expected credit losses |  | (3,933) |  | $ (5,869) | 0
Amortized cost |  | 17,955,515 |  |  | 16,598,808
Cost |  | 2,038,244 |  |  | 957,283
Cost or Amortized Cost |  | 20,056,421 |  |  | 17,923,471
Fixed maturities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value | [1] | 18,515,637 |  |  | 17,263,036
Gross Unrealized Gains | [1] | 595,170 |  |  | 350,303
Gross Unrealized Losses | [1] | (93,777) |  |  | (53,455)
Allowance for expected credit losses | [1],[2] | (3,933) |  |  |
Amortized cost | [1] | 18,018,177 |  |  | 16,966,188
Fixed maturities | Corporate bonds |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 7,965,599 |  |  | 6,406,591
Gross Unrealized Gains | [1] | 390,839 |  |  | 191,889
Gross Unrealized Losses | [1] | (40,448) |  |  | (12,793)
Allowance for expected credit losses | [1],[2] | (1,742) |  |  |
Amortized cost | [1] | 7,616,950 |  |  | 6,227,495
Fixed maturities | Mortgage backed securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 717,081 |  |  | 562,309
Gross Unrealized Gains | [1] | 12,456 |  |  | 9,669
Gross Unrealized Losses | [1] | (3,373) |  |  | (931)
Allowance for expected credit losses | [1],[2] | (336) |  |  |
Amortized cost | [1] | 708,334 |  |  | 553,571
Fixed maturities | Municipal bonds |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 527,410 |  |  | 881,926
Gross Unrealized Gains | [1] | 26,980 |  |  | 24,628
Gross Unrealized Losses | [1] | (877) |  |  | (2,213)
Allowance for expected credit losses | [1],[2] | (62) |  |  |
Amortized cost | [1] | 501,369 |  |  | 859,511
Fixed maturities | Commercial mortgage backed securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 374,360 |  |  | 733,108
Gross Unrealized Gains | [1] | 8,523 |  |  | 14,951
Gross Unrealized Losses | [1] | (4,660) |  |  | (2,330)
Allowance for expected credit losses | [1],[2] | (256) |  |  |
Amortized cost | [1] | 370,753 |  |  | 720,487
Fixed maturities | US government and government agencies |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 4,861,349 |  |  | 4,916,592
Gross Unrealized Gains | [1] | 39,350 |  |  | 36,600
Gross Unrealized Losses | [1] | (4,396) |  |  | (10,134)
Allowance for expected credit losses | [1],[2] | 0 |  |  |
Amortized cost | [1] | 4,826,395 |  |  | 4,890,126
Fixed maturities | Non-US government securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 2,302,646 |  |  | 2,078,757
Gross Unrealized Gains | [1] | 86,996 |  |  | 48,549
Gross Unrealized Losses | [1] | (18,294) |  |  | (20,330)
Allowance for expected credit losses | [1],[2] | 0 |  |  |
Amortized cost | [1] | 2,233,944 |  |  | 2,050,538
Fixed maturities | Asset backed securities |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 1,767,192 |  |  | 1,683,753
Gross Unrealized Gains | [1] | 30,026 |  |  | 24,017
Gross Unrealized Losses | [1] | (21,729) |  |  | (4,724)
Allowance for expected credit losses | [1],[2] | (1,537) |  |  |
Amortized cost | [1] | 1,760,432 |  |  | 1,664,460
Short-term investments |  |  |  |  |
Debt Securities, Available-for-sale [Line Items] |  |  |  |  |
Estimated Fair Value |  | 2,039,097 |  |  | 956,546
Gross Unrealized Gains |  | 1,807 |  |  | 811
Gross Unrealized Losses |  | (954) |  |  | (1,548)
Allowance for expected credit losses |  | 0 | [2] | $ 0 | 0
Cost |  | $ 2,038,244 |  |  | $ 957,283
[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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