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, 2016
U.S. Government sponsored agency securities	$	25,149	$	24	$	—		$	25,173
State and municipal securities	307,038		7,025		(53		)	314,010
Corporate debt securities	95,389		2,773		(8,276		)	89,886
Collateralized mortgage obligations	782,018		5,322		(5,265		)	782,075
Mortgage-backed securities	1,169,552		19,454		(338		)	1,188,668
Auction rate securities	106,871		—		(9,545		)	97,326
Total debt securities	2,486,017		34,598		(23,477		)	2,497,138
Equity securities	14,228		4,852		(13		)	19,067
Total	$	2,500,245	$	39,450	$	(23,490	)	$	2,516,205



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(in thousands)
December 31, 2015
U.S. Government sponsored agency securities	$	25,154	$	35	$	(53	)	$	25,136
State and municipal securities	256,746		6,019		—			262,765
Corporate debt securities	100,336		2,695		(6,076		)	96,955
Collateralized mortgage obligations	835,439		3,042		(16,972		)	821,509
Mortgage-backed securities	1,154,935		10,104		(6,204		)	1,158,835
Auction rate securities	106,772		—		(8,713		)	98,059
Total debt securities	2,479,382		21,895		(38,018		)	2,463,259
Equity securities	$	14,677	$	6,845	$	(8	)	$	21,514
Total	2,494,059		28,740		(38,026		)	2,484,773

Securities carried at $1.6 billion as of March 31, 2016 and $1.7 billion as of December 31, 2015 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 $18.2 million at March 31, 2016 and $20.6 million at December 31, 2015) and other equity investments (estimated fair value of $892,000 at March 31, 2016 and $914,000 at December 31, 2015).

As of March 31, 2016, the financial institutions stock portfolio had a cost basis of $13.4 million and an estimated fair value of $18.2 million, including an investment in a single financial institution with a cost basis of $7.4 million and an estimated fair value of $9.1 million. The estimated fair value of this investment accounted for 50.3% 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, 2016, by contractual maturity, are shown in the following table. Actual maturities may differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.



		Amortized Cost		Estimated Fair Value
	(in thousands)
Due in one year or less		$	64,096	$	64,475
Due from one year to five years		89,237		91,302
Due from five years to ten years		92,154		95,021
Due after ten years		288,960		275,597
		534,447		526,395
Collateralized mortgage obligations		782,018		782,075
Mortgage-backed securities		1,169,552		1,188,668
Total debt securities		$	2,486,017	$	2,497,138

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



	Gross Realized Gains		Gross Realized Losses		Net Gains (Losses)
Three months ended March 31, 2016	(in thousands)
Equity securities	$	733	$	—	$	733
Debt securities	214		—		214
Total	$	947	$	—	$	947
Three months ended March 31, 2015
Equity securities	$	1,970	$	—	$	1,970
Debt securities	2,175		—		2,175
Total	$	4,145	$	—	$	4,145

The following table presents a summary of the cumulative credit related other-than-temporary impairment charges, recognized as components of earnings, for debt securities held by the Corporation at March 31, 2016 and 2015:



	Three months ended March 31
	2016			2015
	(in thousands)
Balance of cumulative credit losses on debt securities, beginning of period	$	(11,510	)	$	(16,242	)
Reductions for securities sold during the period	—			3,938
Reductions for increases in cash flows expected to be collected that are recognized over the remaining life of the security	—			2
Balance of cumulative credit losses on debt securities, end of period	$	(11,510	)	$	(12,302	)

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, 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
	(in thousands)
State and municipal securities	$	17,961	$	(53	)	$	—	$	—		$	17,961	$	(53	)
Corporate debt securities	—		—			30,762		(8,276		)	30,762		(8,276		)
Collateralized mortgage obligations	21,078		(87		)	461,807		(5,178		)	482,885		(5,265		)
Mortgage-backed securities	98,180		(175		)	30,243		(163		)	128,423		(338		)
Auction rate securities	—		—			97,326		(9,545		)	97,326		(9,545		)
Total debt securities	137,219		(315		)	620,138		(23,162		)	757,357		(23,477		)
Equity securities	50		(3		)	12		(10		)	62		(13		)
	$	137,269	$	(318	)	$	620,150	$	(23,172	)	$	757,419	$	(23,490	)

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. Because the decline in market value of these securities is attributable to changes in interest rates and not credit quality, and because 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, the Corporation does not consider these investments to be other-than-temporarily impaired as of March 31, 2016.

As of March 31, 2016, all of the auction rate securities (auction rate certificates, or "ARCs"), were rated above investment grade, with approximately $5.5 million, or 6%, "AAA" rated and $91.8 million, or 94%, "AA" rated. All of the loans underlying the ARCs have principal payments which are guaranteed by the federal government. As of March 31, 2016, all ARCs were current and making scheduled interest payments. Based on management’s evaluations, ARCs with an estimated fair value of $97.3 million were not subject to any other-than-temporary impairment charges as of March 31, 2016. 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.

For its investments in equity securities, particularly its investments in stocks of financial institutions, management evaluates the near-term prospects of the issuers in relation to the severity and duration of the impairment. Based on that evaluation and the Corporation’s ability and intent to hold those investments for a reasonable period of time sufficient for a recovery of fair value, the Corporation does not consider those investments with unrealized holding losses as of March 31, 2016 to be other-than-temporarily impaired.

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, 2016				December 31, 2015
	Amortized cost		Estimated fair value		Amortized cost		Estimated fair value
	(in thousands)
Single-issuer trust preferred securities	$	43,673	$	35,851	$	44,648	$	39,106
Subordinated debt	47,681		49,294		51,653		53,108
Pooled trust preferred securities	—		706		—		706
Corporate debt securities issued by financial institutions	91,354		85,851		96,301		92,920
Other corporate debt securities	4,035		4,035		4,035		4,035
Available for sale corporate debt securities	$	95,389	$	89,886	$	100,336	$	96,955

Single-issuer trust preferred securities had an unrealized loss of $7.8 million at March 31, 2016. Six of the 19 single-issuer trust preferred securities were rated below investment grade by at least one ratings agency, with an amortized cost of $11.5 million and an estimated fair value of $9.0 million at March 31, 2016. All of the single-issuer trust preferred securities rated below investment grade were rated "BB" or "Ba". Two single-issuer trust preferred securities with an amortized cost of $3.7 million and an estimated fair value of $2.4 million at March 31, 2016 were not rated by any ratings agency.

Based on management’s evaluations, corporate debt securities with a fair value of $89.9 million were not subject to any other-than-temporary impairment charges as of March 31, 2016. 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.