Investment Securities

The following tables present the amortized costs, unrealized gains, unrealized losses and approximate fair values of investment securities at December 31, 2017 and 2016:

December 31, 2017



(in thousands)	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
AVAILABLE FOR SALE:
U.S. Treasury and agencies	$	40,021	$	—	$	(323	)	$	39,698
Obligations of states and political subdivisions	303,352		6,206		(1,102		)	308,456
Residential mortgage-backed securities and collateralized mortgage obligations	2,703,997		2,039		(40,391		)	2,665,645
Investments in mutual funds and other equity securities	51,959		11		—			51,970
	$	3,099,329	$	8,256	$	(41,816	)	$	3,065,769
HELD TO MATURITY:
Residential mortgage-backed securities and collateralized mortgage obligations	$	3,803	$	1,103	$	—		$	4,906
	$	3,803	$	1,103	$	—		$	4,906

December 31, 2016



(in thousands)	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
AVAILABLE FOR SALE:
Obligations of states and political subdivisions	$	305,708	$	5,526	$	(3,537	)	$	307,697
Residential mortgage-backed securities and collateralized mortgage obligations	2,428,387		3,664		(40,498		)	2,391,553
Investments in mutual funds and other equity securities	1,959		11		—			1,970
	$	2,736,054	$	9,201	$	(44,035	)	$	2,701,220
HELD TO MATURITY:
Residential mortgage-backed securities and collateralized mortgage obligations	$	4,216	$	1,001	$	—		$	5,217
	$	4,216	$	1,001	$	—		$	5,217

Investment securities that were in an unrealized loss position as of December 31, 2017 and December 31, 2016 are presented in the following tables, based on the length of time individual securities have been in an unrealized loss position. In the opinion of management, these securities are considered only temporarily impaired due to increases in market interest rates or the widening of market spreads subsequent to the initial purchase of the securities, and not due to concerns regarding the underlying credit of the issuers or the underlying collateral.

December 31, 2017



(in thousands)	Less than 12 Months				12 Months or Longer				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
AVAILABLE FOR SALE:
U.S. Treasury and agencies	$	39,699	$	323	$	—	$	—	$	39,699	$	323
Obligations of states and political subdivisions	20,566		322		24,798		780		45,364		1,102
Residential mortgage-backed securities and collateralized mortgage obligations	1,184,000		10,368		1,226,364		30,023		2,410,364		40,391
Total temporarily impaired securities	$	1,244,265	$	11,013	$	1,251,162	$	30,803	$	2,495,427	$	41,816

December 31, 2016



(in thousands)	Less than 12 Months				12 Months or Longer				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
AVAILABLE FOR SALE:
Obligations of states and political subdivisions	$	71,571	$	3,065	$	1,828	$	472	$	73,399	$	3,537
Residential mortgage-backed securities and collateralized mortgage obligations	1,855,304		35,981		182,804		4,517		2,038,108		40,498
Total temporarily impaired securities	$	1,926,875	$	39,046	$	184,632	$	4,989	$	2,111,507	$	44,035

The unrealized losses on U.S. treasury and agencies securities are due to increases in market interest rates and are not due to the underlying credit of the issuers. The unrealized losses on obligations of states and political subdivisions were caused by changes in market interest rates or the widening of market spreads subsequent to the initial purchase of these securities. Management monitors the published credit ratings of these securities for material rating or outlook changes. As of December 31, 2017, 95% of these securities were rated A3/A- or higher by rating agencies. Substantially all of the Company's obligations of states and political subdivisions are general obligation issuances. All of the available for sale residential mortgage-backed securities and collateralized mortgage obligations portfolio in an unrealized loss position at December 31, 2017 are issued or guaranteed by government sponsored enterprises. The unrealized losses on residential mortgage-backed securities and collateralized mortgage obligations were caused by changes in market interest rates or the widening of market spreads subsequent to the initial purchase of these securities, and not concerns regarding the underlying credit of the issuers or the underlying collateral. It is expected that these securities will be settled at a price at least equal to the amortized cost of each investment.

Because the decline in fair value is attributable to changes in interest rates or widening market spreads and not credit quality, and because the Bank does not intend to sell the securities and it is not more likely than not that the Bank will be required to sell these securities before recovery of their amortized cost basis, which may include holding each security until maturity, these investments are not considered other-than-temporarily impaired.

The following table presents the contractual maturities of investment securities at December 31, 2017:



(in thousands)	Available For Sale				Held To Maturity
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
AMOUNTS MATURING IN:
Due within one year	$	2,302	$	2,313	$	—	$	—
Due after one year through five years	94,866		95,169		—		—
Due after five years through ten years	431,773		431,832		19		19
Due after ten years	2,518,429		2,484,485		3,784		4,887
Other investment securities	51,959		51,970		—		—
	$	3,099,329	$	3,065,769	$	3,803	$	4,906

The following table presents the gross realized gains and losses on the sale of securities available for sale for the years ended December 31, 2017, 2016 and 2015:



(in thousands)	2017				2016				2015
	Gains		Losses		Gains		Losses		Gains		Losses
U.S. Treasury and agencies	$	—	$	—	$	—	$	—	$	13	$	—
Obligations of states and political subdivisions	—		9		971		—		631		—
Residential mortgage-backed securities and collateralized mortgage obligations	135		99		270		383		3,119		841
	$	135	$	108	$	1,241	$	383	$	3,763	$	841

The following table presents, as of December 31, 2017, investment securities which were pledged to secure borrowings, public deposits, and repurchase agreements as permitted or required by law:



(in thousands)	Amortized Cost		Fair Value
To Federal Home Loan Bank to secure borrowings	$	453	$	459
To state and local governments to secure public deposits	1,108,242		1,104,046
Other securities pledged principally to secure repurchase agreements	429,549		423,465
Total pledged securities	$	1,538,244	$	1,527,970