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			Balance at End of Period
Year Ended December 31, 2017
Allowance for doubtful accounts	$	17	$	174	$	(163	)	$	28
Deferred tax asset valuation allowance	$	882	$	192	$	—		$	1,074
Insurance reserves	$	712	$	1,687	$	(403	)	$	1,996
Year Ended December 31, 2018
Allowance for doubtful accounts	$	28	$	208	$	(202	)	$	34
Deferred tax asset valuation allowance	$	1,074	$	227	$	(7	)	$	1,294
Insurance reserves	$	1,996	$	1,578	$	(637	)	$	2,937
Year Ended December 31, 2019
Allowance for doubtful accounts	$	34	$	195	$	(195	)	$	34
Deferred tax assets valuation allowance	$	1,294	$	8,616	$	(55	)	$	9,855
Insurance reserves	$	2,937	$	1,451	$	(970	)	$	3,418

⁽¹⁾Additions to insurance reserves include $318 million, $(74) million and $9 million for the years ended December 31, 2017, 2018 and 2019 respectively, for changes in estimates resulting from new developments in prior period claims.

⁽²⁾For the year ended December 31, 2019, the increase in 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 loss from operations, and tax credits generated during the year.