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Reserve for Loss and Loss Expense
 9 Months Ended
Sep. 30, 2021
Insurance Loss Reserves [Abstract]  
Reserve for Loss and Loss Expense Reserve for Loss and Loss Expense
The table below provides a roll forward of reserve for loss and loss expense balances:
Nine Months ended September 30,
($ in thousands)20212020
Gross reserve for loss and loss expense, at beginning of year$4,260,355 4,067,163 
Less: reinsurance recoverable on unpaid loss and loss expense, at beginning of year1
554,269 547,066 
Net reserve for loss and loss expense, at beginning of year3,706,086 3,520,097 
Incurred loss and loss expense for claims occurring in the:  
Current year1,408,240 1,295,285 
Prior years(67,947)(43,210)
Total incurred loss and loss expense1,340,293 1,252,075 
Paid loss and loss expense for claims occurring in the:  
Current year430,288 421,981 
Prior years675,782 646,974 
Total paid loss and loss expense1,106,070 1,068,955 
Net reserve for loss and loss expense, at end of period3,940,309 3,703,217 
Add: Reinsurance recoverable on unpaid loss and loss expense, at end of period670,031 590,059 
Gross reserve for loss and loss expense at end of period$4,610,340 4,293,276 
1 Nine Months 2020 includes an adjustment of $2.9 million related to our adoption of ASU 2016-13, Financial Instruments - Credit Losses.

Prior year reserve development in Nine Months 2021 was favorable by $67.9 million, which included $66.0 million of casualty reserve development. The favorable casualty reserve development included $29.0 million in our general liability line of business, $28.0 million in our workers compensation line of business, $7.0 million in our Excess and Surplus ("E&S") casualty lines of business, and $2.0 million in our businessowners' policies line of business.

Prior year reserve development in Nine Months 2020 was favorable by $43.2 million, which included $50.0 million of casualty reserve development, partially offset by $6.8 million of unfavorable property reserve development. The favorable casualty reserve development included $40.0 million in our workers compensation line of business and $20 million in our general liability line of business, partially offset by $10.0 million of unfavorable reserve development in our commercial automobile line of business.