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Schedule III - Supplementary Insurance Information (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |  |
Deferred Acquisition Costs |  | $ 790,708 | $ 633,400 | $ 569,574
Reserves for Losses and Loss Adjustment Expenses |  | 16,513,928 | 13,891,842 | 11,853,297
Unearned Premiums |  | 4,838,965 | 4,339,549 | 3,753,636
Net Premiums Earned |  | 6,991,935 | 5,786,498 | 5,231,975
Net Losses and Loss Adjustment Expenses Incurred |  | 4,689,599 | 3,133,452 | 2,890,106
Amortization of Deferred Acquisition Costs |  | 1,004,842 | 840,945 | 805,135
Other Operating Expenses | [1] | 875,176 | 800,997 | 677,809
Net Premiums Written |  | 7,437,716 | 6,039,067 | 5,346,747
Other |  |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |  |
Deferred Acquisition Costs |  | 53,705 | 64,044 | 80,858
Reserves for Losses and Loss Adjustment Expenses |  | 1,519,583 | 1,263,629 | 1,032,760
Unearned Premiums |  | 407,714 | 438,907 | 390,114
Net Premiums Earned |  | 560,351 | 556,689 | 578,862
Net Losses and Loss Adjustment Expenses Incurred |  | 440,482 | 453,135 | 441,255
Amortization of Deferred Acquisition Costs |  | 98,071 | 105,980 | 125,558
Other Operating Expenses | [1] | 55,810 | 51,651 | 37,889
Net Premiums Written |  | 537,589 | 532,862 | 604,175
Underwriting segments | Insurance |  |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |  |
Deferred Acquisition Costs |  | 254,833 | 188,684 | 152,360
Reserves for Losses and Loss Adjustment Expenses |  | 8,989,930 | 7,900,328 | 7,093,018
Unearned Premiums |  | 2,334,225 | 1,991,496 | 1,549,183
Net Premiums Earned |  | 2,871,420 | 2,397,080 | 2,205,661
Net Losses and Loss Adjustment Expenses Incurred |  | 2,092,453 | 1,615,475 | 1,520,680
Amortization of Deferred Acquisition Costs |  | 418,483 | 361,614 | 349,702
Other Operating Expenses | [1] | 489,153 | 454,770 | 364,138
Net Premiums Written |  | 3,162,907 | 2,641,726 | 2,212,125
Underwriting segments | Reinsurance |  |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |  |
Deferred Acquisition Costs |  | 278,422 | 197,856 | 166,276
Reserves for Losses and Loss Adjustment Expenses |  | 5,027,742 | 4,270,013 | 3,215,909
Unearned Premiums |  | 1,356,983 | 971,776 | 710,774
Net Premiums Earned |  | 2,162,229 | 1,466,389 | 1,261,216
Net Losses and Loss Adjustment Expenses Incurred |  | 1,628,320 | 1,011,329 | 846,882
Amortization of Deferred Acquisition Costs |  | 354,048 | 239,032 | 211,280
Other Operating Expenses | [1] | 168,011 | 141,484 | 133,350
Net Premiums Written |  | 2,457,370 | 1,602,723 | 1,372,572
Underwriting segments | Mortgage |  |  |  |
SEC Schedule, 12-16, Insurance Companies, Supplementary Insurance Information [Line Items] |  |  |  |
Deferred Acquisition Costs |  | 203,748 | 182,816 | 170,080
Reserves for Losses and Loss Adjustment Expenses |  | 976,673 | 457,872 | 511,610
Unearned Premiums |  | 740,043 | 937,370 | 1,103,565
Net Premiums Earned |  | 1,397,935 | 1,366,340 | 1,186,236
Net Losses and Loss Adjustment Expenses Incurred |  | 528,344 | 53,513 | 81,289
Amortization of Deferred Acquisition Costs |  | 134,240 | 134,319 | 118,595
Other Operating Expenses | [1] | 162,202 | 153,092 | 142,432
Net Premiums Written |  | $ 1,279,850 | $ 1,261,756 | $ 1,157,875
[1] | Certain other operating expenses relate to the Company’s corporate segment (non-underwriting). Such amounts are not reflected in the table above. See note 4, “Segment Information,” to our consolidated financial statements in Item 8 for information related to the corporate segment.

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