Investment Securities

The following tables present the amortized costs, unrealized gains, unrealized losses and approximate fair values of investment securities at September 30, 2018 and December 31, 2017:



(in thousands)	September 30, 2018
	Amortized Cost		Unrealized Gains		Unrealized Losses			Fair Value
AVAILABLE FOR SALE:
U.S. Treasury and agencies	$	40,007	$	—	$	(538	)	$	39,469
Obligations of states and political subdivisions	297,720		1,894		(4,950		)	294,664
Residential mortgage-backed securities and collateralized mortgage obligations	2,635,720		966		(106,425		)	2,530,261
	$	2,973,447	$	2,860	$	(111,913	)	$	2,864,394
HELD TO MATURITY:
Residential mortgage-backed securities and collateralized mortgage obligations	$	3,672	$	1,056	$	—		$	4,728
	$	3,672	$	1,056	$	—		$	4,728



(in thousands)	December 31, 2017
	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 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

For periods presented after December 31, 2017, equity securities are no longer classified as available for sale securities, and are instead separately disclosed on the balance sheet. As of December 31, 2017, the equity securities were reported in investments in mutual funds and other securities within available for sale investment securities.

Investment securities that were in an unrealized loss position as of September 30, 2018 and December 31, 2017 are presented in the following tables, based on the length of time individual securities have been in an unrealized loss position.



(in thousands)	September 30, 2018
	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	$	—	$	—	$	39,469	$	538	$	39,469	$	538
Obligations of states and political subdivisions	115,065		2,819		27,164		2,131		142,229		4,950
Residential mortgage-backed securities and collateralized mortgage obligations	864,126		24,044		1,567,671		82,381		2,431,797		106,425
Total temporarily impaired securities	$	979,191	$	26,863	$	1,634,304	$	85,050	$	2,613,495	$	111,913



(in thousands)	December 31, 2017
	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

The unrealized losses on the available for sale securities portfolio were caused by changes in market interest rates or the widening of market spreads subsequent to the initial purchase of these securities and are not due to the underlying credit of the issuers. Management monitors the securities within the portfolio, including the published credit ratings of the obligations of state and political subdivisions securities for material rating or outlook changes. As of September 30, 2018, 97% of the obligations of states and political subdivisions 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 September 30, 2018 are issued or guaranteed by government sponsored enterprises. It is expected that the mortgage-backed securities and collateralized mortgage obligations securities will be settled at a price at least equal to the amortized cost of each investment.

Because the decline in fair value of the available for sale securities portfolio 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 September 30, 2018:



(in thousands)	Available For Sale				Held To Maturity
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
Due within one year	$	21,795	$	21,726	$	—	$	—
Due after one year through five years	84,666		83,809		—		—
Due after five years through ten years	403,741		394,425		16		16
Due after ten years	2,463,245		2,364,434		3,656		4,712
	$	2,973,447	$	2,864,394	$	3,672	$	4,728

The following table presents, as of September 30, 2018, 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 state and local governments to secure public deposits	$	826,564	$	800,530
Other securities pledged principally to secure repurchase agreements	428,445		410,372
Total pledged securities	$	1,255,009	$	1,210,902