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, 2017
U.S. Government sponsored agency securities	$	5,960	$	94	$	—		$	6,054
State and municipal securities	405,035		3,726		(7,125		)	401,636
Corporate debt securities	88,887		2,531		(2,426		)	88,992
Collateralized mortgage obligations	546,847		1,399		(9,739		)	538,507
Residential mortgage-backed securities	1,218,257		5,782		(10,974		)	1,213,065
Commercial mortgage-backed securities	121,012		390		(335		)	121,067
Auction rate securities	107,410		—		(9,487		)	97,923
Total debt securities	2,493,408		13,922		(40,086		)	2,467,244
Equity securities	10,487		10,968		—			21,455
Total	$	2,503,895	$	24,890	$	(40,086	)	$	2,488,699



	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 $1.7 billion and $1.8 billion as of June 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 $20.5 million at June 30, 2017 and $23.5 million at December 31, 2016) and other equity investments (estimated fair value of $1.0 million at both June 30, 2017 and December 31, 2016).

As of June 30, 2017, the financial institutions stock portfolio had a cost basis of $9.7 million and an estimated fair value of $20.5 million, including an investment in a single financial institution with a cost basis of $4.3 million and an estimated fair value of $8.8 million. The estimated fair value of this investment accounted for 42.9% 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 June 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		$	20,103	$	20,336
Due from one year to five years		27,967		28,335
Due from five years to ten years		111,883		113,050
Due after ten years		447,339		432,884
		607,292		594,605
Residential mortgage-backed securities		1,218,257		1,213,065
Commercial mortgage-backed securities		121,012		121,067
Collateralized mortgage obligations		546,847		538,507
Total debt securities		$	2,493,408	$	2,467,244

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, 2017	(in thousands)
Equity securities	$	1,305	$	—		$	1,305
Debt securities	145		(14		)	131
Total	$	1,450	$	(14	)	$	1,436
Three months ended June 30, 2016
Equity securities	$	4	$	(10	)	$	(6	)
Debt securities	108		(26		)	82
Total	$	112	$	(36	)	$	76

Six months ended June 30, 2017
Equity securities	$	2,350	$	—		$	2,350
Debt securities	206		(14		)	192
Total	$	2,556	$	(14	)	$	2,542
Six months ended June 30, 2016
Equity securities	$	737	$	(10	)	$	727
Debt securities	322		(26		)	296
Total	$	1,059	$	(36	)	$	1,023

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, 2017 and June 30, 2016 was $10.0 million. There were no other-than-temporary impairment charges recognized for the three and six months ended June 30, 2017 and June 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 June 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
June 30, 2017	(in thousands)
State and municipal securities	$	214,497	$	(7,125	)	$	—	$	—		$	214,497	$	(7,125	)
Corporate debt securities	4,020		(9		)	32,148		(2,417		)	36,168		(2,426		)
Collateralized mortgage obligations	144,523		(3,057		)	231,936		(6,682		)	376,459		(9,739		)
Residential mortgage-backed securities	896,206		(10,974		)	—		—			896,206		(10,974		)
Commercial mortgage-backed securities	68,546		(335		)	—		—			68,546		(335		)
Auction rate securities	—		—			97,923		(9,487		)	97,923		(9,487		)
Total debt securities	$	1,327,792	$	(21,500	)	$	362,007	$	(18,586	)	$	1,689,799	$	(40,086	)



	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 June 30, 2017.

As of June 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 June 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 as of June 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:



	June 30, 2017				December 31, 2016
	Amortized cost		Estimated fair value		Amortized cost		Estimated fair value
	(in thousands)
Single-issuer trust preferred securities	$	43,795	$	42,478	$	43,746	$	39,829
Subordinated debt	29,069		29,697		46,231		46,723
Senior debt	12,035		12,407		18,037		18,433
Pooled trust preferred securities	—		422		—		422
Corporate debt securities issued by financial institutions	84,899		85,004		108,014		105,407
Other corporate debt securities	3,988		3,988		4,002		4,002
Available for sale corporate debt securities	$	88,887	$	88,992	$	112,016	$	109,409

Single-issuer trust preferred securities had an unrealized loss of $1.3 million at June 30, 2017. Six of the 19 single-issuer trust preferred securities, with an amortized cost of $11.6 million and an estimated fair value of $10.9 million at June 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 June 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 as of June 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.