Securities Available for Sale

The amortized cost and approximate fair value of investments in securities, all of which are classified as available for sale according to management’s intent, are summarized as follows:



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(dollars in thousands)
As of December 31, 2017
U.S. Treasury securities	$	228,302	$	2	$	(527	)	$	227,777
Mortgage-backed securities:
Government National Mortgage Association	481,441		94		(9,330		)	472,205
Federal Home Loan Mortgage Corporation	203,561		—		(2,869		)	200,692
Federal National Mortgage Association	161,958		—		(2,528		)	159,430
Small Business Assistance Program	237,965		212		(1,838		)	236,339
States and political subdivision securities	70,034		86		(943		)	69,177
Other	1,006		15		—			1,021
Total	$	1,384,267	$	409	$	(18,035	)	$	1,366,641



	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Estimated Fair Value
	(dollars in thousands)
As of September 30, 2017
U.S. Treasury securities	$	228,039	$	579	$	(15	)	$	228,603
Mortgage-backed securities:
Government National Mortgage Association	511,457		228		(6,635		)	505,050
Federal Home Loan Mortgage Corporation	169,147		75		(1,247		)	167,975
Federal National Mortgage Association	170,247		22		(1,287		)	168,982
Small Business Assistance Program	224,005		726		(1,001		)	223,730
States and political subdivision securities	73,041		187		(642		)	72,586
Other	1,006		28		—			1,034
Total	$	1,376,942	$	1,845	$	(10,827	)	$	1,367,960

The amortized cost and approximate fair value of debt securities available for sale as of December 31, 2017 and September 30, 2017, by contractual maturity, are shown below. Maturities of mortgage-backed securities may differ from contractual maturities because the mortgages underlying the securities may be called or repaid without penalty.



	December 31, 2017				September 30, 2017
	Amortized Cost		Estimated Fair Value		Amortized Cost		Estimated Fair Value
	(dollars in thousands)
Due in one year or less	$	93,551	$	93,461	$	91,535	$	91,597
Due after one year through five years	189,128		188,080		193,117		193,373
Due after five years through ten years	15,535		15,291		16,306		16,097
Due after ten years	122		122		122		122
	298,336		296,954		301,080		301,189
Mortgage-backed securities	1,084,925		1,068,666		1,074,856		1,065,737
Securities without contractual maturities	1,006		1,021		1,006		1,034
Total	$	1,384,267	$	1,366,641	$	1,376,942	$	1,367,960

Proceeds from sales of securities available for sale were $0.2 million for the three months ended December 31, 2017 and $0.0 million for the three months ended December 31, 2016. No gross gains (pre-tax) or gross losses (pre-tax) were realized on the sales for the three months ended December 31, 2017 and 2016 using the specific identification method. The Company recognized no other-than-temporary impairment for the three months ended December 31, 2017 and 2016.

Securities with an estimated fair value of approximately $990.6 million and $951.4 million at December 31, 2017 and September 30, 2017, respectively, were pledged as collateral on public deposits, securities sold under agreements to repurchase, and for other purposes as required or permitted by law. The counterparties do not have the right to sell or pledge the securities the Company has pledged as collateral.

As detailed in the following tables, certain investments in debt securities, which are approximately 90% and 68% of the Company’s investment portfolio at estimated fair value at December 31, 2017 and September 30, 2017, respectively, are reported in the consolidated financial statements at an amount less than their amortized cost. Based on evaluation of available evidence, including recent changes in market interest rates, credit rating information, implicit or explicit government guarantees, and information obtained from regulatory filings, management believes the declines in fair value of these securities are temporary. As the Company does not intend to sell the securities and it is not more likely than not the Company will be required to sell the securities before the recovery of their amortized cost basis, which may be maturity, the Company does not consider the securities to be other-than-temporarily impaired at December 31, 2017 or September 30, 2017.

The following table presents the Company’s gross unrealized losses and approximate fair value in investments, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position:



	Less than 12 months					12 months or more					Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
	(dollars in thousands)
As of December 31, 2017
U.S. Treasury securities	$	183,651	$	(415	)	$	19,133	$	(112	)	$	202,784	$	(527	)
Mortgage-backed securities	184,096		(1,703		)	787,256		(14,862		)	971,352		(16,565		)
States and political subdivision securities	5,198		(33		)	54,253		(910		)	59,451		(943		)
Total	$	372,945	$	(2,151	)	$	860,642	$	(15,884	)	$	1,233,587	$	(18,035	)



	Less than 12 months					12 months or more					Total
	Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses			Estimated Fair Value		Unrealized Losses
	(dollars in thousands)
As of September 30, 2017
U.S. Treasury securities	$	10,003	$	(15	)	$	—	$	—		$	10,003	$	(15	)
Mortgage-backed securities	$	635,969	$	(5,425	)	$	241,368	$	(4,746	)	$	877,337	$	(10,171	)
States and political subdivision securities	21,705		(197		)	25,773		(444		)	47,478		(641		)
Total	$	667,677	$	(5,637	)	$	267,141	$	(5,190	)	$	934,818	$	(10,827	)

As of December 31, 2017 and September 30, 2017, the Company had 320 and 249 securities, respectively, in an unrealized loss position.