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)
March 31, 2018
U.S. Government sponsored agency securities	$	21,105	$	40	$	(183	)	$	20,962
State and municipal securities	413,632		2,247		(9,780		)	406,099
Corporate debt securities	101,367		2,482		(1,894		)	101,955
Collateralized mortgage obligations	695,840		236		(16,376		)	679,700
Residential mortgage-backed securities	1,084,582		3,006		(33,422		)	1,054,166
Commercial mortgage-backed securities	231,378		15		(4,501		)	226,892
Auction rate securities	107,410		—		(4,361		)	103,049
Total	$	2,655,314	$	8,026	$	(70,517	)	$	2,592,823



	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

Securities carried at $1.8 billion at March 31, 2018 and 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 March 31, 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		$	14,055	$	14,063
Due from one year to five years		36,871		36,986
Due from five years to ten years		119,069		119,529
Due after ten years		473,519		461,487
		643,514		632,065
Residential mortgage-backed securities⁽¹⁾		1,084,582		1,054,166
Commercial mortgage-backed securities⁽¹⁾		231,378		226,892
Collateralized mortgage obligations⁽¹⁾		695,840		679,700
Total		$	2,655,314	$	2,592,823
(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 on the sales of equity and debt securities:



	Gross Realized Gains
Three months ended March 31, 2018	(in thousands)
Equity securities	$	9
Debt securities	10
Total	$	19
Three months ended March 31, 2017
Equity securities	$	1,045
Debt securities	61
Total	$	1,106

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 March 31, 2018 and March 31, 2017 was $11.5 million. There were no other-than-temporary impairment charges recognized for the three months ended March 31, 2018 and March 31, 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 March 31, 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
March 31, 2018	(in thousands)
U.S. Government sponsored agency securities	$	12,499	$	(183	)	$	—	$	—		$	12,499	$	(183	)
State and municipal securities	163,345		(3,132		)	114,024		(6,648		)	277,369		(9,780		)
Corporate debt securities	9,466		(130		)	28,219		(1,764		)	37,685		(1,894		)
Collateralized mortgage obligations	456,200		(7,184		)	176,076		(9,192		)	632,276		(16,376		)
Residential mortgage-backed securities	527,502		(13,611		)	471,703		(19,811		)	999,205		(33,422		)
Commercial mortgage-backed securities	196,887		(3,884		)	21,539		(617		)	218,426		(4,501		)
Auction rate securities	—		—			103,049		(4,361		)	103,049		(4,361		)
Total	$	1,365,899	$	(28,124	)	$	914,610	$	(42,393	)	$	2,280,509	$	(70,517	)



	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 March 31, 2018.

As of March 31, 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 months ended March 31, 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:



	March 31, 2018				December 31, 2017
	Amortized cost		Estimated fair value		Amortized cost		Estimated fair value
	(in thousands)
Single-issuer trust preferred securities	$	31,352	$	30,513	$	31,335	$	30,703
Subordinated debt	54,011		54,357		49,013		49,533
Senior debt	12,029		12,245		12,031		12,392
Pooled trust preferred securities	—		865		—		707
Corporate debt securities issued by financial institutions	97,392		97,980		92,379		93,335
Other corporate debt securities	3,975		3,975		3,974		3,974
Available for sale corporate debt securities	$	101,367	$	101,955	$	96,353	$	97,309

Single-issuer trust preferred securities had an unrealized loss of $839,000 at March 31, 2018. Four of the 17 single-issuer trust preferred securities, with an amortized cost of $4.9 million and an estimated fair value of $4.7 million at March 31, 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 $3.1 million at March 31, 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 months ended March 31, 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.