Investment securities (Tables)

12 Months Ended

Dec. 31, 2023

Investments, Debt and Equity Securities [Abstract]

Amortized Cost and Estimated Fair Value of Investment Securities

The amortized cost and estimated fair value of investment securities were as follows:

(Dollars in millions)	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
December 31, 2023
Investment securities available for sale:
U.S. Treasury and federal agencies	$	7,818	$	—	$	113	$	7,705
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		425		—		9		416
Residential		2,272		—		118		2,154
Other debt securities		176		—		11		165
		10,691		—		251		10,440
Investment securities held to maturity:
U.S. Treasury and federal agencies		1,005		—		31		974
Obligations of states and political subdivisions		2,501		—		67		2,434
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		2,033		—		130		1,903
Residential		9,747		4		802		8,949
Privately issued		42		9		5		46
Other debt securities		2		—		—		2
		15,330		13		1,035		14,308
Total debt securities	$	26,021	$	13	$	1,286	$	24,748
Equity and other securities:
Readily marketable equity — at fair value	$	266	$	5	$	3	$	268
Other — at cost		859		—		—		859
Total equity and other securities	$	1,125	$	5	$	3	$	1,127
December 31, 2022
Investment securities available for sale:
U.S. Treasury and federal agencies	$	7,914	$	—	$	243	$	7,671
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		595		—		21		574
Residential		2,501		—		171		2,330
Other debt securities		183		1		10		174
		11,193		1		445		10,749
Investment securities held to maturity:
U.S. Treasury and federal agencies		1,054		—		46		1,008
Obligations of states and political subdivisions		2,577		—		117		2,460
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		912		—		103		809
Residential		8,935		1		891		8,045
Privately issued		50		9		8		51
Other debt securities		2		—		—		2
		13,530		10		1,165		12,375
Total debt securities	$	24,723	$	11	$	1,610	$	23,124
Equity and other securities:
Readily marketable equity — at fair value	$	153	$	2	$	4	$	151
Other — at cost		781		—		—		781
Total equity and other securities	$	934	$	2	$	4	$	932

Investment Ratings of All Privately Issued Mortgage-Backed Securities and Other Debt Securities

As of December 31, 2023, the latest available investment ratings of all obligations of states and political subdivisions, privately issued mortgage-backed securities and other debt securities were:

					Average Credit Rating of Fair Value Amount
(Dollars in millions)	Amortized Cost		Estimated Fair Value		A or Better		BBB		BB		B or Less		Not Rated
Obligations of states and     political subdivisions	$	2,501	$	2,434	$	2,426	$	1	$	—	$	—	$	7
Privately issued mortgage-     backed securities		42		46		—		—		—		1		45
Other debt securities		178		167		8		73		31		—		55

Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity

At December 31, 2023, the amortized cost and estimated fair value of debt securities by contractual maturity were as follows:

(Dollars in millions)	Amortized Cost		Estimated Fair Value
Debt securities available for sale:
Due in one year or less	$	5,023	$	4,970
Due after one year through five years		2,921		2,856
Due after five years through ten years		50		44
Due after ten years		—		—
		7,994		7,870
Mortgage-backed securities		2,697		2,570
	$	10,691	$	10,440
Debt securities held to maturity:
Due in one year or less	$	15	$	14
Due after one year through five years		1,179		1,147
Due after five years through ten years		1,351		1,327
Due after ten years		963		922
		3,508		3,410
Mortgage-backed securities		11,822		10,898
	$	15,330	$	14,308

Investment Securities in Continuous Unrealized Loss Position

A summary of investment securities that as of December 31, 2023 and 2022 had been in a continuous unrealized loss position for less than twelve months and those that had been in a continuous unrealized loss position for twelve months or longer follows:

	Less Than 12 Months				12 Months or More
(Dollars in millions)	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
December 31, 2023
Investment securities available for sale:
U.S. Treasury and federal agencies	$	229	$	1	$	7,474	$	112
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		74		1		330		8
Residential		151		2		1,959		116
Other debt securities		6		—		154		11
		460		4		9,917		247
Investment securities held to maturity:
U.S. Treasury and federal agencies		50		—		924		31
Obligations of states and political subdivisions		218		3		2,172		64
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		328		9		1,575		121
Residential		955		11		7,139		791
Privately issued		—		—		34		5
		1,551		23		11,844		1,012
Total	$	2,011	$	27	$	21,761	$	1,259
December 31, 2022
Investment securities available for sale:
U.S. Treasury and federal agencies	$	6,707	$	184	$	842	$	59
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		574		21		—		—
Residential		2,296		169		29		2
Other debt securities		93		4		73		6
		9,670		378		944		67
Investment securities held to maturity:
U.S. Treasury and federal agencies		1,008		46		—		—
Obligations of states and political subdivisions		2,449		117		—		—
Mortgage-backed securities:
Government issued or guaranteed:
Commercial		809		103		—		—
Residential		6,293		619		1,319		272
Privately issued		—		—		36		8
		10,559		885		1,355		280
Total	$	20,229	$	1,263	$	2,299	$	347