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)
September 30, 2017
U.S. Government sponsored agency securities	$	5,961	$	54	$	—		$	6,015
State and municipal securities	415,313		4,005		(5,405		)	413,913
Corporate debt securities	92,355		2,578		(1,956		)	92,977
Collateralized mortgage obligations	601,845		1,380		(9,547		)	593,678
Residential mortgage-backed securities	1,184,797		5,850		(8,561		)	1,182,086
Commercial mortgage-backed securities	161,960		299		(627		)	161,632
Auction rate securities	107,410		—		(9,254		)	98,156
Total debt securities	2,569,641		14,166		(35,350		)	2,548,457
Equity securities	6,560		6,499		—			13,059
Total	$	2,576,201	$	20,665	$	(35,350	)	$	2,561,516



	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
Residential mortgage-backed securities	1,328,192		6,546		(16,900		)	1,317,838
Commercial mortgage-backed securities	25,100		—		(537		)	24,563
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 $1.9 billion and $1.8 billion as of September 30, 2017 and December 31, 2016, respectively, were pledged as collateral to secure public and trust deposits and customer repurchase agreements.

Equity securities include common stocks of publicly traded financial institutions (estimated fair value of $12.1 million at September 30, 2017 and $23.5 million at December 31, 2016) and other equity investments (estimated fair value of $1.0 million at both September 30, 2017 and December 31, 2016).

As of September 30, 2017, the financial institutions stock portfolio had a cost basis of $5.8 million and an estimated fair value of $12.1 million, including an investment in a single financial institution with a cost basis of $4.2 million and an estimated fair value of $8.8 million. The estimated fair value of this investment accounted for 73.4% 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 September 30, 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		$	23,940	$	24,118
Due from one year to five years		30,708		31,196
Due from five years to ten years		114,114		115,336
Due after ten years		452,277		440,411
		621,039		611,061
Residential mortgage-backed securities		1,184,797		1,182,086
Commercial mortgage-backed securities		161,960		161,632
Collateralized mortgage obligations		601,845		593,678
Total debt securities		$	2,569,641	$	2,548,457

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 September 30, 2017	(in thousands)
Equity securities	$	4,817	$	—		$	4,817
Debt securities	12		(232		)	(220		)
Total	$	4,829	$	(232	)	$	4,597
Three months ended September 30, 2016
Equity securities	$	2	$	—		$	2
Debt securities	—		—			—
Total	$	2	$	—		$	2

Nine months ended September 30, 2017
Equity securities	$	7,167	$	—		$	7,167
Debt securities	218		(246		)	(28		)
Total	$	7,385	$	(246	)	$	7,139
Nine months ended September 30, 2016
Equity securities	$	739	$	(10	)	$	729
Debt securities	322		(26		)	296
Total	$	1,061	$	(36	)	$	1,025

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 September 30, 2017 and September 30, 2016 was $10.0 million. There were no other-than-temporary impairment charges recognized for the three and nine months ended September 30, 2017 and September 30, 2016.

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 September 30, 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
September 30, 2017	(in thousands)
State and municipal securities	$	121,527	$	(1,930	)	$	87,466	$	(3,475	)	$	208,993	$	(5,405	)
Corporate debt securities	3,570		(16		)	31,533		(1,940		)	35,103		(1,956		)
Collateralized mortgage obligations	85,335		(837		)	301,009		(8,710		)	386,344		(9,547		)
Residential mortgage-backed securities	796,019		(8,359		)	5,513		(202		)	801,532		(8,561		)
Commercial mortgage-backed securities	87,260		(627		)	—		—			87,260		(627		)
Auction rate securities	—		—			98,156		(9,254		)	98,156		(9,254		)
Total debt securities	$	1,093,711	$	(11,769	)	$	523,677	$	(23,581	)	$	1,617,388	$	(35,350	)



	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		)
Residential mortgage-backed securities	1,112,947		(16,900		)	—		—			1,112,947		(16,900		)
Commercial mortgage-backed securities	24,563		(537		)	—		—			24,563		(537		)
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 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 September 30, 2017.

As of September 30, 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 September 30, 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 for the three and nine months ended September 30, 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:



	September 30, 2017				December 31, 2016
	Amortized cost		Estimated fair value		Amortized cost		Estimated fair value
	(in thousands)
Single-issuer trust preferred securities	$	39,186	$	38,251	$	43,746	$	39,829
Subordinated debt	37,147		37,859		46,231		46,723
Senior debt	12,033		12,456		18,037		18,433
Pooled trust preferred securities	—		422		—		422
Corporate debt securities issued by financial institutions	88,366		88,988		108,014		105,407
Other corporate debt securities	3,989		3,989		4,002		4,002
Available for sale corporate debt securities	$	92,355	$	92,977	$	112,016	$	109,409

Single-issuer trust preferred securities had an unrealized loss of $935,000 at September 30, 2017. Five of the 18 single-issuer trust preferred securities, with an amortized cost of $6.9 million and an estimated fair value of $6.6 million at September 30, 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 September 30, 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 for the three and nine months ended September 30, 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.