AOCI is comprised of items that have not been recognized in earnings but may be recognized in earnings in the future when certain events occur. Changes in each component for the three and nine months ended September 30, 2021 and 2020 were as follows:


Three Months Ended September 30, 2021 (Millions), net of tax	Net Unrealized Gains (Losses) on Debt Securities		Foreign Currency Translation Adjustment Gains (Losses)		Net Unrealized Pension and Other Postretirement Benefit Gains (Losses)		Accumulated Other Comprehensive Income (Loss)
Balances as of June 30, 2021	$	42	$	(2,227)	$	(696)	$	(2,881)
Net unrealized losses	(8)		—		—		(8)
Net translation on investments in foreign operations	—		(238)		—		(238)
Net hedges of investments in foreign operations	—		155		—		155
Pension and other postretirement benefits	—		—		9		9
Net change in accumulated other comprehensive income (loss)	(8)		(83)		9		(82)
Balances as of September 30, 2021	$	34	$	(2,310)	$	(687)	$	(2,963)


Nine Months Ended September 30, 2021 (Millions), net of tax	Net Unrealized Gains (Losses) on Debt Securities		Foreign Currency Translation Adjustment Gains (Losses)		Net Unrealized Pension and Other Postretirement Benefit Gains (Losses)		Accumulated Other Comprehensive Income (Loss)
Balances as of December 31, 2020	$	65	$	(2,229)	$	(731)	$	(2,895)
Net unrealized losses	(31)		—		—		(31)
Net translation on investments in foreign operations	—		(134)		—		(134)
Net hedges of investments in foreign operations	—		53		—		53
Pension and other postretirement benefits	—		—		44		44
Net change in accumulated other comprehensive income (loss)	(31)		(81)		44		(68)
Balances as of September 30, 2021	$	34	$	(2,310)	$	(687)	$	(2,963)


Three Months Ended September 30, 2020 (Millions), net of tax	Net Unrealized Gains (Losses) on Debt Securities		Foreign Currency Translation Adjustment Gains (Losses)		Net Unrealized Pension and Other Postretirement Benefit Gains (Losses)		Accumulated Other Comprehensive Income (Loss)
Balances as of June 30, 2020	$	85	$	(2,388)	$	(608)	$	(2,911)
Net unrealized losses	(9)		—		—		(9)

Net translation on investments in foreign operations	—		210		—		210
Net hedges of investments in foreign operations	—		(170)		—		(170)
Pension and other postretirement benefits	—		—		8		8
Net change in accumulated other comprehensive income (loss)	(9)		40		8		39
Balances as of September 30, 2020	$	76	$	(2,348)	$	(600)	$	(2,872)


Nine Months Ended September 30, 2020 (Millions), net of tax	Net Unrealized Gains (Losses) on Debt Securities		Foreign Currency Translation Adjustment Gains (Losses)		Net Unrealized Pension and Other Postretirement Benefit Gains (Losses)		Accumulated Other Comprehensive Income (Loss)
Balances as of December 31, 2019	$	33	$	(2,189)	$	(581)	$	(2,737)
Net unrealized gains	43		—		—		43
Decrease due to amounts reclassified into earnings	—		(3)		—		(3)
Net translation on investments in foreign operations	—		(379)		—		(379)
Net hedges of investments in foreign operations	—		223		—		223
Pension and other postretirement benefits	—		—		(19)		(19)
Net change in accumulated other comprehensive income (loss)	43		(159)		(19)		(135)
Balances as of September 30, 2020	$	76	$	(2,348)	$	(600)	$	(2,872)

The following table shows the tax impact for the three and nine months ended September 30 for the changes in each component of AOCI presented above:


	Tax expense (benefit)
	Three Months Ended September 30,				Nine Months Ended September 30,
(Millions)	2021		2020		2021		2020
Net unrealized (losses) gains on debt securities	$	(3)	$	(3)	$	(9)	$	13
Net translation on investments in foreign operations	(2)		(14)		3		10
Net hedges of investments in foreign operations	47		(57)		12		67
Pension and other postretirement benefits	5		(1)		18		10
Total tax impact	$	47	$	(75)	$	24	$	100