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Schedule IV - Reinsurance (Details) - USD ($) $ in Thousands | 3 Months Ended | 12 Months Ended
--- | --- | ---
Dec. 31, 2020 | Sep. 30, 2020 | Jun. 30, 2020 | Mar. 31, 2020 | Dec. 31, 2019 | Sep. 30, 2019 | Jun. 30, 2019 | Mar. 31, 2019 | Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gross Amount |  |  |  |  |  |  |  |  |  | $ 6,553,910 | $ 5,681,523 | $ 4,838,902
Ceded to other Companies |  |  |  |  |  |  |  |  |  | (2,650,352) | (2,099,893) | (1,614,257)
Assumed From Other Companies |  |  |  |  |  |  |  |  |  | 3,534,158 | 2,457,437 | 2,122,102
Net Amount |  | $ 1,758,015 | $ 1,874,144 | $ 1,668,311 | $ 2,137,246 | $ 1,455,453 | $ 1,613,457 | $ 1,444,898 | $ 1,525,259 | $ 7,437,716 | $ 6,039,067 | $ 5,346,747
Percentage of Amount Assumed to Net |  |  |  |  |  |  |  |  |  | 47.50% | 40.70% | 39.70%
Other |  |  |  |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gross Amount |  |  |  |  |  |  |  |  |  | $ 396,743 | $ 339,169 | $ 253,760
Ceded to other Companies | [1] |  |  |  |  |  |  |  |  | (190,957) | (222,019) | (130,840)
Assumed From Other Companies | [1] |  |  |  |  |  |  |  |  | 331,803 | 415,712 | 481,255
Net Amount |  |  |  |  |  |  |  |  |  | $ 537,589 | $ 532,862 | $ 604,175
Percentage of Amount Assumed to Net |  |  |  |  |  |  |  |  |  | 61.70% | 78.00% | 79.70%
Underwriting segments |  |  |  |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Ceded to other Companies |  |  |  |  |  |  |  |  |  | $ (2,732,564) | $ (2,189,440) | $ (1,791,851)
Net Amount |  |  |  |  |  |  |  |  |  | 6,900,127 | 5,506,205 | 4,742,572
Underwriting segments | Insurance |  |  |  |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gross Amount |  |  |  |  |  |  |  |  |  | 4,659,416 | 3,879,752 | 3,232,234
Ceded to other Companies | [1] |  |  |  |  |  |  |  |  | (1,525,655) | (1,266,267) | (1,050,207)
Assumed From Other Companies | [1] |  |  |  |  |  |  |  |  | 29,146 | 28,241 | 30,098
Net Amount |  |  |  |  |  |  |  |  |  | $ 3,162,907 | $ 2,641,726 | $ 2,212,125
Percentage of Amount Assumed to Net |  |  |  |  |  |  |  |  |  | 0.90% | 1.10% | 1.40%
Underwriting segments | Reinsurance |  |  |  |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gross Amount |  |  |  |  |  |  |  |  |  | $ 305,435 | $ 238,229 | $ 213,809
Ceded to other Companies | [1] |  |  |  |  |  |  |  |  | (1,014,716) | (720,500) | (539,950)
Assumed From Other Companies | [1] |  |  |  |  |  |  |  |  | 3,166,651 | 2,084,994 | 1,698,713
Net Amount |  |  |  |  |  |  |  |  |  | $ 2,457,370 | $ 1,602,723 | $ 1,372,572
Percentage of Amount Assumed to Net |  |  |  |  |  |  |  |  |  | 128.90% | 130.10% | 123.80%
Underwriting segments | Mortgage |  |  |  |  |  |  |  |  |  |  |  |
SEC Schedule, 12-17, Insurance Companies, Reinsurance [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Gross Amount |  |  |  |  |  |  |  |  |  | $ 1,192,316 | $ 1,224,373 | $ 1,139,099
Ceded to other Companies | [1] |  |  |  |  |  |  |  |  | (194,149) | (204,509) | (202,833)
Assumed From Other Companies | [1] |  |  |  |  |  |  |  |  | 281,683 | 241,892 | 221,609
Net Amount |  |  |  |  |  |  |  |  |  | $ 1,279,850 | $ 1,261,756 | $ 1,157,875
Percentage of Amount Assumed to Net |  |  |  |  |  |  |  |  |  | 22.00% | 19.20% | 19.10%
[1] | Certain amounts included in the gross premiums written of each segment are related to intersegment transactions and are included in the gross premiums written of each segment. Accordingly, the sum of gross premiums written for each segment does not agree to the total gross premiums written as shown in the table above due to the elimination of intersegment transactions in the total.

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