This table provides detailed information about securities available for sale at December 31, 2020 and 2019 (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Fair
2020	Cost		Gains		Losses			Value
U.S. Treasury	$	29,911	$	829	$	—		$	30,740
U.S. Agencies		89,554		6,395		—			95,949
Mortgage-backed		5,266,394		202,944		(1,157	)		5,468,181
State and political subdivisions		3,424,309		199,848		(538	)		3,623,619
Corporates		77,566		3,649		(16	)		81,199
Total	$	8,887,734	$	413,665	$	(1,711	)	$	9,299,688

			Gross		Gross
	Amortized		Unrealized		Unrealized			Fair
2019	Cost		Gains		Losses			Value
U.S. Treasury	$	63,835	$	408	$	(165	)	$	64,078
U.S. Agencies		89,867		3,154		—			93,021
Mortgage-backed		4,030,688		58,184		(17,078	)		4,071,794
State and political subdivisions		2,954,276		78,867		(3,226	)		3,029,917
Corporates		185,314		3,259		(21	)		188,552
Total	$	7,323,980	$	143,872	$	(20,490	)	$	7,447,362

The following table presents contractual maturity information for securities available for sale at December 31, 2020 (in thousands):

	Amortized		Fair
	Cost		Value
Due in 1 year or less	$	246,086	$	247,233
Due after 1 year through 5 years		592,484		612,069
Due after 5 years through 10 years		636,566		667,001
Due after 10 years		2,146,204		2,305,204
Total		3,621,340		3,831,507
Mortgage-backed securities		5,266,394		5,468,181
Total securities available for sale	$	8,887,734	$	9,299,688

Securities may be disposed of before contractual maturities due to sales by the Company or because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

Proceeds from the sales of securities available for sale were $315.9 million, $412.0 million, and $95.5 million for 2020, 2019, and 2018, respectively. Securities transactions resulted in gross realized gains of $7.3 million for

2020, $4.6 million for 2019, and $581 thousand for 2018. The gross realized losses were $274 thousand for 2020, $1.4 million for 2019, and $3 thousand for 2018.

Securities available for sale with a fair value of $7.8 billion and $5.8 billion at December 31, 2020 and December 31, 2019, respectively, were pledged to secure U.S. Government deposits, other public deposits, certain trust deposits, derivative transactions, and repurchase agreements. Of these amounts, securities with a fair value of $371.5 million at December 31, 2020 and $481.2 million at December 31, 2019 were pledged at the Federal Reserve Discount Window but were unencumbered as of those dates.

Accrued interest on securities available for sale totaled $42.6 million as of December 31, 2020 and is included in the Accrued income line on the Company’s Consolidated Balance Sheets. The total amount of accrued interest is excluded from the amortized cost of available securities presented above. Further, the Company has elected not to measure an ACL for accrued interest receivable.

The following table shows the Company’s available-for-sale investments’ gross unrealized losses and fair value, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2020 and 2019 (in thousands):

	Less than 12 months							12 months or more							Total
2020	Count		Fair Value		Unrealized Losses			Count		Fair Value		Unrealized Losses			Count		Fair Value		Unrealized Losses
Description of Securities
U.S. Treasury		—	$	—	$	—			—	$	—	$	—			—	$	—	$	—
U.S. Agencies		—		—		—			—		—		—			—		—		—
Mortgage-backed		16		174,234		(1,157	)		—		—		—			16		174,234		(1,157	)
State and political subdivisions		24		55,653		(279	)		6		2,833		(259	)		30		58,486		(538	)
Corporates		4		5,335		(16	)		—		—		—			4		5,335		(16	)
Total		44	$	235,222	$	(1,452	)		6	$	2,833	$	(259	)		50	$	238,055	$	(1,711	)

	Less than 12 months					12 months or more					Total
2019	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
Description of Securities
U.S. Treasury	$	—	$	—		$	19,863	$	(165	)	$	19,863	$	(165	)
U.S. Agencies		—		—			—		—			—		—
Mortgage-backed		947,415		(5,236	)		517,824		(11,842	)		1,465,239		(17,078	)
State and political subdivisions		361,440		(3,084	)		27,501		(142	)		388,941		(3,226	)
Corporates		13,685		(21	)		—		—			13,685		(21	)
Total	$	1,322,540	$	(8,341	)	$	565,188	$	(12,149	)	$	1,887,728	$	(20,490	)

The unrealized losses in the Company’s investments in GSE mortgage-backed securities, State and political subdivisions, and Corporates were caused by changes in interest rates, and not from a decline in credit of the underlying issuers. The U.S. Treasury, U.S. Agency, and GSE mortgage-backed securities are all considered to be agency-backed securities with no risk of loss as they are either explicitly or implicitly guaranteed by the U.S. government. The changes in fair value in the agency-backed portfolios are solely driven by change in interest rates caused by changing economic conditions. The Company has no knowledge of any underlying credit issues and the cash flows underlying the debt securities have not changed and are not expected to be impacted by changes in interest rates.

For the State and political subdivision portfolio, the majority of the Company’s holdings are in general obligation bonds, which have a very low historical default rate due to issuers generally having unlimited taxing authority to service the debt. For both the State and political and Corporate portfolios, the Company has a robust process for monitoring credit risk, including both pre-purchase and ongoing post-purchase credit reviews and analysis. The Company monitors credit ratings of all bond issuers in these segments and reviews available financial data, including market and sector trends.

As of December 31, 2020, there is no ACL related to the Company’s available-for-sale securities as the decline in fair value did not result from credit issues.

The following table shows the Company’s held-to-maturity investments’ amortized cost, fair value, and gross unrealized gains and losses at December 31, 2020 and 2019, respectively (in thousands):

				Gross		Gross
	Amortized			Unrealized		Unrealized			Fair
2020	Cost			Gains		Losses			Value
State and political subdivisions:
Due in 1 year or less	$	4,907		$	29	$	—		$	4,936
Due after 1 year through 5 years		123,643			3,402		(144	)		126,901
Due after 5 years through 10 years		423,759			12,845		(1,566	)		435,038
Due after 10 years		462,305			13,447		(13,183	)		462,569
Total state and political subdivisions	$	1,014,614		$	29,723	$	(14,893	)	$	1,029,444
Allowance for credit losses		(2,610	)
Total state and political subdivisions, net of allowance for credit losses	$	1,012,004

			Gross		Gross
	Amortized		Unrealized		Unrealized			Fair
2019	Cost		Gains		Losses			Value
State and political subdivisions:
Due in 1 year or less	$	15,323	$	5	$	(60	)	$	15,268
Due after 1 year through 5 years		100,623		374		(699	)		100,298
Due after 5 years through 10 years		394,591		389		(8,400	)		386,580
Due after 10 years		605,565		494		(25,860	)		580,199
Total state and political subdivisions	$	1,116,102	$	1,262	$	(35,019	)	$	1,082,345

Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

The following table shows the Company’s held-to-maturity investments’ gross unrealized losses and fair value, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2020 and 2019 (in thousands):

	Less than 12 months					12 months or more					Total
2020	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
State and political subdivisions	$	132,271	$	(4,591	)	$	109,712	$	(10,302	)	$	241,983	$	(14,893	)
Total	$	132,271	$	(4,591	)	$	109,712	$	(10,302	)	$	241,983	$	(14,893	)

	Less than 12 months					12 months or more					Total
2019	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
State and political subdivisions	$	471,544	$	(12,424	)	$	546,572	$	(22,595	)	$	1,018,116	$	(35,019	)
Total	$	471,544	$	(12,424	)	$	546,572	$	(22,595	)	$	1,018,116	$	(35,019	)

The unrealized losses in the Company’s held-to-maturity portfolio were caused by changes in the interest rate environment. The underlying bonds are evaluated for credit losses in conjunction with management’s estimate of the ACL based on credit rating.

The following table shows the amortized cost basis by credit rating of the Company’s held-to-maturity investments at December 31, 2020 (in thousands):

	Amortized Cost Basis by Credit Rating - HTM Debt Securities
2020	A		BBB		BB		CCC-C		Total
State and political subdivisions:
Competitive	$	340,290	$	558,786	$	18,078	$	8,135	$	925,289
Utilities		56,232		33,093		—		—		89,325
Total state and political subdivisions	$	396,522	$	591,879	$	18,078	$	8,135	$	1,014,614

Competitive held-to-maturity securities include not-for-profit enterprises that provide public functions such as housing, higher education or healthcare, but do so in a competitive environment. It also includes project financings that can have relatively high enterprise risk, such as deals backed by revenues from sports or convention facilities or start-up transportation revenues.

Utilities are public enterprises providing essential services with a monopoly or near-monopoly over the service area. This includes environmental utilities (water, sewer, solid waste), power utilities (electric distribution and generation, gas), and transportation utilities (airports, parking, toll roads, mass transit, ports).

All held-to-maturity securities were current and not past due at December 31, 2020.

Accrued interest on securities held to maturity totaled $5.3 million as of December 31, 2020 and is included in the Accrued income line on the Company’s Consolidated Balance Sheets. The total amount of accrued interest is excluded from the amortized cost of available-for-sale securities presented above. Further, the Company has elected not to measure an ACL for accrued interest receivable.

The net unrealized gain on trading securities at December 31, 2020 was $13 thousand. The net unrealized gain on trading securities at December 31, 2019 was $1 thousand and the net unrealized loss on trading securities at December 31, 2018 was $18 thousand. Net unrealized gains/losses are included in trading and investment banking income on the Consolidated Statements of Income. Securities sold not yet purchased totaled $2.2 million and $14.6 million at December 31, 2020 and 2019, respectively, and are classified within the Other liabilities line of the Company’s Consolidated Balance Sheets.

The table below provides detailed information for Other securities at December 31, 2020 and 2019 (in thousands):

Investment in FRB stock is based on the capital structure of the investing bank, and investment in FHLB stock is mainly tied to the level of borrowings from the FHLB. These holdings are carried at cost. Equity securities with readily determinable fair values are generally traded on an exchange and market prices are readily available. Equity securities without readily determinable fair values include PCM alternative investments in hedge funds and private equity funds, which are accounted for as equity-method investments. Also included in this category are equity investments which are held by a subsidiary qualified as a Small Business Investment Company, as well as investments in low-income housing partnerships within the areas the Company serves. Unrealized gains or losses on equity securities with and without readily determinable fair values are recognized in the Investment securities gains, net line of the Company’s Consolidated Statements of Income.

The following table presents the components of Investment securities gains, net for the years ended December 31, 2020, 2019, and 2018 (in thousands):

	Year Ended December 31,
	2020			2019			2018
Investment securities gains, net
Available-for-sale debt securities:
Gains realized on sales	$	7,255		$	4,636		$	581
Losses realized on sales		(274	)		(1,418	)		(3	)
Equity securities with readily determinable fair values:
Fair value adjustments, net		110,768			—			—
Equity securities without readily determinable fair values:
Fair value adjustments, net		2,885			(973	)		2,943
Total investment securities gains, net	$	120,634		$	2,245		$	3,521