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

March 31, 2020	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
U.S. Treasury	$	30,549	$	1,056	$	—		$	31,605
U.S. Agencies		94,195		6,421		—			100,616
Mortgage-backed		4,067,786		171,441		(4,578	)		4,234,649
State and political subdivisions		2,980,676		114,292		(1,276	)		3,093,692
Corporates		179,974		1,368		(2,453	)		178,889
Total	$	7,353,180	$	294,578	$	(8,307	)	$	7,639,451

December 31, 2019	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair 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 March 31, 2020 (in thousands):

	Amortized		Fair
	Cost		Value
Due in 1 year or less	$	225,943	$	226,438
Due after 1 year through 5 years		842,108		852,880
Due after 5 years through 10 years		580,312		594,772
Due after 10 years		1,637,031		1,730,712
Total		3,285,394		3,404,802
Mortgage-backed securities		4,067,786		4,234,649
Total securities available for sale	$	7,353,180	$	7,639,451

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.

For the three months ended March 31, 2020, proceeds from the sales of securities available for sale were $84.4 million compared to $53.3 million for the same period in 2019. Securities transactions resulted in gross realized gains of $1.2 million and $809 thousand for the three months ended March 31, 2020 and 2019, respectively. There were no gross realized losses for either the three months ended March 31, 2020 or 2019.

Securities available for sale with a fair value of $6.0 billion at March 31, 2020 and $5.8 billion at December 31, 2019 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 market value of $979.2 million and $481.2 million at March 31, 2020 and December 31, 2019, respectively, were pledged at the Federal Reserve Discount Window but were unencumbered as of those dates.

Accrued interest on securities available for sale totaled $32.5 million as of March 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 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 March 31, 2020 and December 31, 2019 (in thousands):

	Less than 12 months							12 months or more							Total
March 31, 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		1		4,406		—			—		—		—			1		4,406		—
Mortgage-backed		36		159,593		(4,079	)		10		34,227		(499	)		46		193,820		(4,578	)
State and political subdivisions		184		127,235		(1,267	)		4		1,158		(9	)		188		128,393		(1,276	)
Corporates		63		96,594		(2,453	)		—		—		—			63		96,594		(2,453	)
Total		284	$	387,828	$	(7,799	)		14	$	35,385	$	(508	)		298	$	423,213	$	(8,307	)

	Less than 12 months					12 months or more					Total
December 31, 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 U.S. Treasury securities, U.S. Agency securities, Government Sponsored Entity (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 subdivision 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 March 31, 2020, the Company does not believe the decline in value in these portfolios is related to credit impairments and instead is due to declining interest rates. The Company does not have the intent to sell these securities and does not believe it is more likely than not that the Company will be required to sell these securities before a recovery of amortized cost. As of March 31, 2020, there is no ACL related to the Company’s AFS 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 March 31, 2020 and December 31, 2019, respectively (in thousands):

				Gross		Gross
	Amortized			Unrealized		Unrealized			Fair
March 31, 2020	Cost			Gains		Losses			Value
State and political subdivisions:
Due in 1 year or less	$	14,151		$	20	$	—		$	14,171
Due after 1 year through 5 years		93,532			2,785		(135	)		96,182
Due after 5 years through 10 years		414,223			11,534		(760	)		424,997
Due after 10 years		592,254			22,230		(2,925	)		611,559
Total state and political subdivisions	$	1,114,160		$	36,569	$	(3,820	)	$	1,146,909
Allowance for credit losses		(3,235	)
Total state and political subdivisions, net of allowance for credit losses	$	1,110,925

			Gross		Gross
	Amortized		Unrealized		Unrealized			Fair
December 31, 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.

There were no sales of securities held to maturity during the three months ended March 31, 2020 or 2019.

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

	Less than 12 months					12 months or more					Total
March 31, 2020	Fair Value		Unrealized Losses			Fair Value		Unrealized Losses			Fair Value		Unrealized Losses
State and political subdivisions	$	20,357	$	(384	)	$	113,935	$	(3,436	)	$	134,292	$	(3,820	)
Total	$	20,357	$	(384	)	$	113,935	$	(3,436	)	$	134,292	$	(3,820	)

	Less than 12 months					12 months or more					Total
December 31, 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 March 31, 2020 (in thousands):

	Amortized Cost Basis by Credit Rating - HTM Debt Securities
March 31, 2020	AA		A		BBB		BB		B		CCC-C		Total
State and political subdivisions:
Competitive	$	8,998	$	394,082	$	597,984	$	22,519	$	685	$	12,576	$	1,036,844
Utilities		—		57,349		19,967		—		—		—		77,316
Total state and political subdivisions	$	8,998	$	451,431	$	617,951	$	22,519	$	685	$	12,576	$	1,114,160

Competitive held-to-maturity securities include not-for-profit enterprises that provide an 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 ventures.

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 March 31, 2020.

Accrued interest on securities held to maturity totaled $6.5 million as of March 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.

There were net unrealized gains on trading securities of $150 thousand and $52 thousand at March 31, 2020 and March 31, 2019, respectively. Net unrealized gains/losses are included in trading and investment banking income on the Company’s Consolidated Statements of Income. Securities sold not yet purchased totaled $17.6 million and $14.6 million at March 31, 2020 and December 31, 2019, respectively, and are classified within the Other liabilities line of the Company’s Consolidated Balance Sheets.

The table below provides detailed information for FRB stock and Federal Home Loan Bank (FHLB) stock and other securities at March 31, 2020 and December 31, 2019 (in thousands):

			Gross		Gross
	Amortized		Unrealized		Unrealized			Fair
March 31, 2020	Cost		Gains		Losses			Value
FRB and FHLB stock	$	33,252	$	—	$	—		$	33,252
Other securities – non-marketable		99,543		2,428		(29	)		101,942
Total Other securities	$	132,795	$	2,428	$	(29	)	$	135,194

			Gross		Gross
	Amortized		Unrealized		Unrealized			Fair
December 31, 2019	Cost		Gains		Losses			Value
FRB and FHLB stock	$	33,262	$	—	$	—		$	33,262
Other securities – non-marketable		69,868		5,295		(5	)		75,158
Total Other securities	$	103,130	$	5,295	$	(5	)	$	108,420

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. Other non-marketable securities include Prairie Capital Management (PCM) alternative investments in hedge funds and private equity funds, which are accounted for as equity-method investments. Also included in other non-marketable securities 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. The fair value of other non-marketable securities includes alternative investment securities of $5.6 million at March 31, 2020 and $7.0 million at December 31, 2019. Unrealized gains or losses on alternative investments are recognized in the Other noninterest income line on the Company’s Consolidated Statements of Income.