
	Three Months Ended
	March 31
(in millions of U.S. dollars)	2021		2020
Gross unpaid losses and loss expenses – beginning of period	$	67,811	$	62,690
Reinsurance recoverable on unpaid losses – beginning of period (1)	(14,647)		(14,181)
Net unpaid losses and loss expenses – beginning of period	53,164		48,509


Net losses and loss expenses incurred in respect of losses occurring in:
Current year	5,249		4,605
Prior years (2)	(196)		(120)
Total	5,053		4,485
Net losses and loss expenses paid in respect of losses occurring in:
Current year	732		920
Prior years	3,161		3,335
Total	3,893		4,255
Foreign currency revaluation and other	71		(565)
Net unpaid losses and loss expenses – end of period	54,395		48,174
Reinsurance recoverable on unpaid losses (1)	14,860		14,040
Gross unpaid losses and loss expenses – end of period	$	69,255	$	62,214

(2) Relates to prior period loss reserve development only and excludes prior period development related to reinstatement premiums, expense adjustments and earned premiums totaling $4 million and $2 million for the three months ended March 31, 2021 and 2020, respectively.

Gross and net unpaid losses and loss expenses increased $1,444 million and $1,231 million, respectively, for the three months ended March 31, 2021, reflecting an increase in underlying exposure due to premium growth and catastrophe losses incurred, partially offset by favorable prior period development.

Prior period development (PPD) arises from changes to loss estimates recognized in the current year that relate to loss events that occurred in previous calendar years and excludes the effect of losses from the development of earned premium from previous accident years. Long-tail lines include lines such as workers' compensation, general liability, and professional liability; while short-tail lines include lines such as most property lines, energy, personal accident, and agriculture.


	Three Months Ended March 31
(in millions of U.S. dollars)		Long-tail		Short-tail		Total
2021
North America Commercial P&C Insurance		$	(46)	$	(81)	$	(127)
North America Personal P&C Insurance		—		(40)		(40)
North America Agricultural Insurance		—		(2)		(2)
Overseas General Insurance		—		(25)		(25)
Global Reinsurance		—		(7)		(7)
Corporate		9		—		9
Total		$	(37)	$	(155)	$	(192)
2020
North America Commercial P&C Insurance		$	(43)	$	(62)	$	(105)
North America Personal P&C Insurance		—		1		1
North America Agricultural Insurance		—		(14)		(14)
Overseas General Insurance		—		(4)		(4)
Global Reinsurance		—		(7)		(7)
Corporate		11		—		11
Total		$	(32)	$	(86)	$	(118)

Significant prior period movements by segment, principally driven by reserve reviews completed during each respective period, are discussed in more detail below. The remaining net development for long-tail lines and short-tail business for each segment and Corporate comprises numerous favorable and adverse movements across a number of lines and accident years, none of which is significant individually or in the aggregate.

For the three months ended March 31, 2021, net favorable PPD was $127 million, which was the net result of several underlying favorable and adverse movements, and was driven by the following principal changes:

•Net favorable development of $51 million in professional liability (errors & omissions and third-party cyber risk), driven by accident years 2016 and 2017, which saw lower than expected loss emergence, partly offset by higher than expected development in accident year 2019;

•Net favorable development of $35 million in voluntary environmental lines, driven by accident years 2017 and prior, which saw lower than expected loss emergence, partly offset by higher than expected development in accident year 2019; and

•Net adverse development of $57 million in excess and umbrella portfolios, with accident years 2015 through 2019 continuing to experience higher than expected loss development, partly offset by favorable development in accident years 2014 and prior.

•Net favorable development of $25 million in property and marine coverages in accident year 2020, driven by lower than expected non-catastrophe loss development; and

•Net adverse development of $41 million in first-party cyber risk, driven by accident years 2019 and 2020, which experienced higher than expected loss development as well as heightened frequency and severity.

For the three months ended March 31, 2020, net favorable PPD was $105 million, which was the net result of several underlying favorable and adverse movements, and was driven by the following principal changes:

•Net favorable development of $66 million in professional liability (errors & omissions and third-party cyber risk), driven by accident years 2016 and prior, which saw lower than expected emergence;

•Net favorable development of $43 million in voluntary environmental lines, driven by accident years 2016 and prior, which saw lower than expected emergence and a favorable revision to loss development patterns;

•Net favorable development of $28 million in construction workers’ compensation, mainly in accident years 2016 and prior, which saw lower than expected reported loss emergence and a favorable revision to loss development patterns;

•Net adverse development of $49 million in excess and umbrella portfolios, with accident years 2015 through 2019 experiencing higher than expected reported development, partially offset by lower than expected emergence in accident years 2014 and prior; and

•Net adverse development of $23 million in wholesale general liability coverages, driven by higher than expected reported loss emergence in accident years 2014 through 2019.

For the three months ended March 31, 2021, net favorable PPD was $40 million, driven by accident year 2020, which experienced better than expected non-catastrophe loss development in homeowners and valuables.

For the three months ended March 31, 2021, net favorable PPD was $25 million, which reflects favorable loss experience across several lines, mainly in accident year 2020, none of which were significant individually or in the aggregate.