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Investments (Available-For-Sale Investments) (Details) - USD ($) $ in Thousands | Jun. 30, 2020 | Dec. 31, 2019
--- | --- | ---
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | $ 9,203,517 | $ 9,448,840
Available-for-sale fixed maturity investments, gross unrealized holding gains | 871,181 | 536,488
Available-for-sale fixed maturity investments, gross unrealized holding losses | (14,024) | (14,419)
Available-for-sale, allowance for credit losses | (202) |
Available-for-sale fixed maturity investments, estimated fair value | 10,060,472 | 9,970,909
Short-term investments, available-for-sale, amortized cost | 931,555 | 1,194,953
Short-term investments, available-for-sale, gross unrealized holding gains | 489 | 1,355
Short-term investments, available-for-sale, gross unrealized holding losses | (2,772) | (60)
Short-term investments, available-for-sale, allowance for credit losses | 0 |
Short-term investments, available-for-sale, estimated fair value | 929,272 | 1,196,248
Investments, available-for-sale, amortized cost | 10,135,072 | 10,643,793
Investments, available-for-sale, gross unrealized holding gains | 871,670 | 537,843
Investments, available-for-sale, gross unrealized holding losses | (16,796) | (14,479)
Investments, available-for-sale, allowance for credit losses | (202) |
Investments, available-for-sale, estimated fair value | 10,989,744 | 11,167,157
U.S. Treasury securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 343,467 | 282,305
Available-for-sale fixed maturity investments, gross unrealized holding gains | 11,447 | 2,883
Available-for-sale fixed maturity investments, gross unrealized holding losses | (19) | (402)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 354,895 | 284,786
U.S. government-sponsored enterprises [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 323,864 | 318,831
Available-for-sale fixed maturity investments, gross unrealized holding gains | 50,944 | 23,949
Available-for-sale fixed maturity investments, gross unrealized holding losses | 0 | (200)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 374,808 | 342,580
Obligations of states, municipalities and political subdivisions [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 3,852,301 | 3,954,779
Available-for-sale fixed maturity investments, gross unrealized holding gains | 354,186 | 235,915
Available-for-sale fixed maturity investments, gross unrealized holding losses | (1,032) | (812)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 4,205,455 | 4,189,882
Foreign governments [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 1,352,962 | 1,415,639
Available-for-sale fixed maturity investments, gross unrealized holding gains | 172,377 | 135,763
Available-for-sale fixed maturity investments, gross unrealized holding losses | (9,943) | (9,398)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 1,515,396 | 1,542,004
Commercial mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 1,730,795 | 1,761,777
Available-for-sale fixed maturity investments, gross unrealized holding gains | 148,218 | 57,450
Available-for-sale fixed maturity investments, gross unrealized holding losses | (35) | (1,382)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 1,878,978 | 1,817,845
Residential mortgage-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 836,066 | 855,641
Available-for-sale fixed maturity investments, gross unrealized holding gains | 71,299 | 32,949
Available-for-sale fixed maturity investments, gross unrealized holding losses | (35) | (517)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 907,330 | 888,073
Asset-backed securities [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 8,122 | 11,042
Available-for-sale fixed maturity investments, gross unrealized holding gains | 159 | 28
Available-for-sale fixed maturity investments, gross unrealized holding losses | 0 | (22)
Available-for-sale, allowance for credit losses | 0 |
Available-for-sale fixed maturity investments, estimated fair value | 8,281 | 11,048
Corporate bonds [Member] |  |
Debt Securities, Available-for-sale [Line Items] |  |
Fixed maturities, available-for-sale amortized cost | 755,940 | 848,826
Available-for-sale fixed maturity investments, gross unrealized holding gains | 62,551 | 47,551
Available-for-sale fixed maturity investments, gross unrealized holding losses | (2,960) | (1,686)
Available-for-sale, allowance for credit losses | (202) |
Available-for-sale fixed maturity investments, estimated fair value | $ 815,329 | $ 894,691

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