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, 2017
U.S. Government sponsored agency securities	$	5,959	$	37	$	—		$	5,996
State and municipal securities	404,125		2,352		(13,853		)	392,624
Corporate debt securities	109,595		2,055		(3,773		)	107,877
Collateralized mortgage obligations	575,596		1,811		(11,521		)	565,886
Mortgage-backed securities	1,321,573		6,155		(14,107		)	1,313,621
Auction rate securities	107,312		—		(9,873		)	97,439
Total debt securities	2,524,160		12,410		(53,127		)	2,483,443
Equity securities	11,416		11,158		—			22,574
Total	$	2,535,576	$	23,568	$	(53,127	)	$	2,506,017



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
December 31, 2016
U.S. Government sponsored agency securities	$	132	$	2	$	—		$	134
State and municipal securities	405,274		2,043		(15,676		)	391,641
Corporate debt securities	112,016		1,978		(4,585		)	109,409
Collateralized mortgage obligations	604,095		1,943		(12,178		)	593,860
Mortgage-backed securities	1,353,292		6,546		(17,437		)	1,342,401
Auction rate securities	107,215		—		(9,959		)	97,256
Total debt securities	2,582,024		12,512		(59,835		)	2,534,701
Equity securities	12,231		12,295		—			24,526
Total	$	2,594,255	$	24,807	$	(59,835	)	$	2,559,227

Securities carried at $1.8 billion as of March 31, 2017 and December 31, 2016 were pledged as collateral to secure public and trust deposits and customer repurchase agreements.

Equity securities include common stocks of financial institutions (estimated fair value of $21.6 million at March 31, 2017 and $23.5 million at December 31, 2016) and other equity investments (estimated fair value of $969,000 at March 31, 2017 and $1.0 million at December 31, 2016).

As of March 31, 2017, the financial institutions stock portfolio had a cost basis of $10.6 million and an estimated fair value of $21.6 million, including an investment in a single financial institution with a cost basis of $5.1 million and an estimated fair value of $9.6 million. The estimated fair value of this investment accounted for 44.6% of the estimated fair value of the Corporation's investments in the common stocks of publicly traded financial institutions. No other investment in a single financial institution in the financial institutions stock portfolio exceeded 10% of the portfolio's estimated fair value.

The amortized cost and estimated fair values of debt securities as of March 31, 2017, 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		$	53,510	$	53,729
Due from one year to five years		27,178		27,785
Due from five years to ten years		113,529		113,708
Due after ten years		432,774		408,714
		626,991		603,936
Collateralized mortgage obligations		575,596		565,886
Mortgage-backed securities		1,321,573		1,313,621
Total debt securities		$	2,524,160	$	2,483,443

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 March 31, 2017	(in thousands)
Equity securities	$	1,045	$	—	$	1,045
Debt securities	61		—		61
Total	$	1,106	$	—	$	1,106
Three months ended March 31, 2016
Equity securities	$	733	$	—	$	733
Debt securities	214		—		214
Total	$	947	$	—	$	947

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, 2017 and March 31, 2016 was $11.5 million. There were no other-than-temporary impairment charges recognized for the three months ended March 31, 2017 and March 31, 2016.

The following tables present 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, 2017 and December 31, 2016:



	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, 2017	(in thousands)
State and municipal securities	$	237,313	$	(13,853	)	$	—	$	—		$	237,313	$	(13,853	)
Corporate debt securities	4,020		(11		)	35,364		(3,762		)	39,384		(3,773		)
Collateralized mortgage obligations	145,052		(3,672		)	246,408		(7,849		)	391,460		(11,521		)
Mortgage-backed securities	1,089,397		(14,107		)	—		—			1,089,397		(14,107		)
Auction rate securities	—		—			97,439		(9,873		)	97,439		(9,873		)
Total debt securities	1,475,782		(31,643		)	379,211		(21,484		)	1,854,993		(53,127		)



	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, 2016	(in thousands)
State and municipal securities	$	247,509	$	(15,676	)	$	—	$	—		$	247,509	$	(15,676	)
Corporate debt securities	11,922		(110		)	34,629		(4,475		)	46,551		(4,585		)
Collateralized mortgage obligations	166,905		(3,899		)	258,237		(8,279		)	425,142		(12,178		)
Mortgage-backed securities	1,137,510		(17,437		)	—		—			1,137,510		(17,437		)
Auction rate securities	—		—			97,256		(9,959		)	97,256		(9,959		)
Total debt securities	1,563,846		(37,122		)	390,122		(22,713		)	1,953,968		(59,835		)

The decline in market 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, 2017.

As of March 31, 2017, all of the auction rate securities (auction rate certificates, or "ARCs"), were rated above investment grade. All of the loans underlying the ARCs have principal payments which are guaranteed by the federal government. As of March 31, 2017, all ARCs were current and making scheduled interest payments, and based on management’s evaluations, were not subject to any other-than-temporary impairment charges as of March 31, 2017. 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 value of corporate debt securities:



	March 31, 2017				December 31, 2016
	Amortized cost		Estimated fair value		Amortized cost		Estimated fair value
	(in thousands)
Single-issuer trust preferred securities	$	43,770	$	40,720	$	43,746	$	39,829
Subordinated debt	43,787		44,399		46,231		46,723
Senior debt	18,036		18,334		18,037		18,433
Pooled trust preferred securities	—		422		—		422
Corporate debt securities issued by financial institutions	105,593		103,875		108,014		105,407
Other corporate debt securities	4,002		4,002		4,002		4,002
Available for sale corporate debt securities	$	109,595	$	107,877	$	112,016	$	109,409

Single-issuer trust preferred securities had an unrealized loss of $3.1 million at March 31, 2017. Six of the 19 single-issuer trust preferred securities, with an amortized cost of $11.6 million and an estimated fair value of $10.3 million at March 31, 2017, 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 "BB" and "Ba". Two single-issuer trust preferred securities with an amortized cost of $3.8 million and an estimated fair value of $2.8 million at March 31, 2017 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 as of March 31, 2017. 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.