Investment Securities

The following table presents the amortized cost and estimated fair values of investment securities, which were all classified as available for sale:



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
June 30, 2018
U.S. Government sponsored agency securities	$	26,830	$	5	$	(309	)	$	26,526
State and municipal securities	415,460		2,101		(9,865		)	407,696
Corporate debt securities	92,411		1,261		(1,802		)	91,870
Collateralized mortgage obligations	742,282		406		(18,366		)	724,322
Residential mortgage-backed securities	1,035,724		2,342		(37,102		)	1,000,964
Commercial mortgage-backed securities	244,046		34		(5,297		)	238,783
Auction rate securities	107,410		—		(4,288		)	103,122
Total	$	2,664,163	$	6,149	$	(77,029	)	$	2,593,283



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
December 31, 2017
U.S. Government sponsored agency securities	$	5,962	$	2	$	(26	)	$	5,938
State and municipal securities	405,860		5,638		(2,549		)	408,949
Corporate debt securities	96,353		2,832		(1,876		)	97,309
Collateralized mortgage obligations	611,927		491		(9,795		)	602,623
Residential mortgage-backed securities	1,132,080		3,957		(15,241		)	1,120,796
Commercial mortgage-backed securities	215,351		—		(2,596		)	212,755
Auction rate securities	107,410		—		(8,742		)	98,668
Total debt securities	2,574,943		12,920		(40,825		)	2,547,038
Equity securities	776		142		—			918
Total	$	2,575,719	$	13,062	$	(40,825	)	$	2,547,956

On August 1, 2018, the Corporation transferred debt securities with an amortized cost of $665.5 million and an estimated fair value of $641.7 million from the available for sale classification to the held to maturity classification. These securities consisted of residential mortgage-backed securities ($505.5 million amortized cost and $485.3 million estimated fair value) and state and municipal securities ($160.0 million amortized cost and $156.4 million estimated fair value). The transfer was accounted for at estimated fair value. These securities were transferred as the Corporation has the positive intent and ability to hold these securities to maturity.

Securities carried at $1.5 billion at June 30, 2018 and $1.8 billion at December 31, 2017, were pledged as collateral to secure public and trust deposits and customer repurchase agreements.

The amortized cost and estimated fair values of debt securities as of June 30, 2018, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities as certain investment securities are subject to call or prepayment with or without call or prepayment penalties.



		Amortized Cost		Estimated Fair Value
	(in thousands)
Due in one year or less		$	7,305	$	7,313
Due from one year to five years		44,956		44,959
Due from five years to ten years		109,639		109,503
Due after ten years		480,211		467,439
		642,111		629,214
Residential mortgage-backed securities⁽¹⁾		1,035,724		1,000,964
Commercial mortgage-backed securities⁽¹⁾		244,046		238,783
Collateralized mortgage obligations⁽¹⁾		742,282		724,322
Total		$	2,664,163	$	2,593,283
(1) Maturities for mortgage-backed securities and collateralized mortgage obligations are dependent upon the interest rate environment and prepayments on the underlying loans.

The following table presents information related to the gross realized gains and losses on the sales of equity and debt securities:



	Gross Realized Gains		Gross Realized Losses			Net Gains (Losses)
Three months ended June 30, 2018	(in thousands)
Equity securities	$	—	$	—		$	—
Debt securities	1,530		(1,526		)	4
Total	$	1,530	$	(1,526	)	$	4
Three months ended June 30, 2017
Equity securities	$	1,305	$	—		$	1,305
Debt securities	145		(14		)	131
Total	$	1,450	$	(14	)	$	1,436

Six months ended June 30, 2018
Equity securities	$	9	$	—		$	9
Debt securities	1,540		(1,526		)	14
Total	$	1,549	$	(1,526	)	$	23
Six months ended June 30, 2017
Equity securities	$	2,350	$	—		$	2,350
Debt securities	206		(14		)	192
Total	$	2,556	$	(14	)	$	2,542

The cumulative balance of credit-related other-than-temporary impairment charges, previously recognized as components of earnings, for debt securities held by the Corporation at June 30, 2018 and June 30, 2017 was $11.5 million. There were no other-than-temporary impairment charges recognized for the three and six months ended June 30, 2018 and June 30, 2017.

The following table presents the gross unrealized losses and estimated fair values of investments, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at June 30, 2018 and December 31, 2017:



	Less than 12 months					12 months or longer					Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
June 30, 2018	(in thousands)
U.S. Government sponsored agency securities	$	24,028	$	(309	)	$	—	$	—		$	24,028	$	(309	)
State and municipal securities	166,323		(3,264		)	113,946		(6,601		)	280,269		(9,865		)
Corporate debt securities	35,112		(220		)	22,554		(1,582		)	57,666		(1,802		)
Collateralized mortgage obligations	529,808		(10,258		)	136,723		(8,108		)	666,531		(18,366		)
Residential mortgage-backed securities	501,230		(15,521		)	450,713		(21,581		)	951,943		(37,102		)
Commercial mortgage-backed securities	182,155		(3,903		)	45,352		(1,394		)	227,507		(5,297		)
Auction rate securities	—		—			103,122		(4,288		)	103,122		(4,288		)
Total	$	1,438,656	$	(33,475	)	$	872,410	$	(43,554	)	$	2,311,066	$	(77,029	)



	Less than 12 months					12 months or longer					Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
December 31, 2017	(in thousands)
U.S. Government sponsored agency securities	$	5,830	$	(26	)	$	—	$	—		$	5,830	$	(26	)
State and municipal securities	11,650		(50		)	118,297		(2,499		)	129,947		(2,549		)
Corporate debt securities	4,544		(48		)	32,163		(1,828		)	36,707		(1,876		)
Collateralized mortgage obligations	303,932		(2,408		)	187,690		(7,387		)	491,622		(9,795		)
Residential mortgage-backed securities	511,378		(4,348		)	500,375		(10,893		)	1,011,753		(15,241		)
Commercial mortgage-backed securities	190,985		(2,118		)	21,770		(478		)	212,755		(2,596		)
Auction rate securities	—		—			98,668		(8,742		)	98,668		(8,742		)
Total	$	1,028,319	$	(8,998	)	$	958,963	$	(31,827	)	$	1,987,282	$	(40,825	)

The Corporation’s collateralized mortgage obligations and mortgage-backed securities have contractual terms that generally do not permit the issuer to settle the securities at a price less than the amortized cost of the investment. The change in fair value of these securities is attributable to changes in interest rates and not credit quality, and the Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost. Therefore, the Corporation does not consider these investments to be other-than-temporarily impaired as of June 30, 2018.

As of June 30, 2018, all of the auction rate securities (auction rate certificates, or "ARCs"), were rated above investment grade. Based on management’s evaluations, none of the ARCs were subject to any other-than-temporary impairment charges for the three and six months ended June 30, 2018. The Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost, which may be at maturity.

The majority of the Corporation's available for sale corporate debt securities are issued by financial institutions. The following table presents the amortized cost and estimated fair values of corporate debt securities:



	June 30, 2018				December 31, 2017
	Amortized cost		Estimated fair value		Amortized cost		Estimated fair value
	(in thousands)
Single-issuer trust preferred securities	$	21,779	$	20,396	$	31,335	$	30,703
Subordinated debt	54,649		54,534		49,013		49,533
Senior debt	12,027		12,109		12,031		12,392
Pooled trust preferred securities	—		875		—		707
Corporate debt securities issued by financial institutions	88,455		87,914		92,379		93,335
Other corporate debt securities	3,956		3,956		3,974		3,974
Available for sale corporate debt securities	$	92,411	$	91,870	$	96,353	$	97,309

Single-issuer trust preferred securities had an unrealized loss of $1.4 million at June 30, 2018. Two of the 11 single-issuer trust preferred securities, with an amortized cost of $2.0 million and an estimated fair value of $1.9 million at June 30, 2018, were rated below investment grade by at least one ratings agency. All of the single-issuer trust preferred securities rated below investment grade were rated either "BB" or "Ba". Two single-issuer trust preferred securities with an amortized cost of $3.8 million and an estimated fair value of $3.2 million at June 30, 2018 were not rated by any ratings agency.

Based on management’s evaluations, no corporate debt securities were subject to any other-than-temporary impairment charges for the three and six months ended June 30, 2018. The Corporation does not have the intent to sell and does not believe it will more likely than not be required to sell any of these securities prior to a recovery of their fair value to amortized cost, which may be at maturity.