The table below details the activity of the allowance for doubtful accounts, deferred tax asset valuation allowance, and insurance reserves (in millions):


	Balance at Beginning of Period		Additions (1), (2)		Deductions (2)		Balance at End of Period
Year Ended December 31, 2019
Allowance for doubtful accounts	$	34	$	195	$	(195)	$	34
Deferred tax asset valuation allowance	$	1,294	$	8,616	$	(55)	$	9,855
Insurance reserves	$	2,937	$	1,451	$	(970)	$	3,418
Year Ended December 31, 2020
Allowance for doubtful accounts	$	34	$	178	$	(157)	$	55
Deferred tax assets valuation allowance	$	9,855	$	3,655	$	(100)	$	13,410
Insurance reserves	$	3,418	$	950	$	(902)	$	3,466
Year Ended December 31, 2021
Allowance for doubtful accounts	$	55	$	246	$	(250)	$	51
Deferred tax assets valuation allowance	$	13,410	$	571	$	(61)	$	13,920
Insurance reserves	$	3,466	$	1,696	$	(1,174)	$	3,988

(1) Additions to insurance reserves include $9 million, $35 million and $69 million for the years ended December 31, 2019, 2020 and 2021 respectively, for changes in estimates resulting from new developments in prior period claims. Additions to insurance reserves also include $374 million for the year ended December 31, 2021 for reserves assumed in connection with a loss portfolio transfer reinsurance agreement. For additional information on the loss portfolio transfer reinsurance agreement, see Note 1 – Description of Business and Summary of Significant Accounting Policies.

(2) For the year ended December 31, 2019, the increase in the valuation allowance was primarily attributable to a step-up in the tax basis of intellectual property rights, an increase in U.S. federal, state and Netherlands deferred tax assets resulting from the loss from operations, and tax credits generated during the year.

For the year ended December 31, 2020, the increase in the valuation allowance was primarily attributable to an increase in tax rate in Netherlands, an increase in U.S. federal, state and Netherlands deferred tax assets resulting from the loss from operations, and tax credits generated during the year.