Securities Available for Sale (Tables)

12 Months Ended

Sep. 30, 2020

Investments, Debt and Equity Securities [Abstract]

Summary of amortized cost and approximate fair value of investments in securities

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 September 30, 2020
U.S. Treasury securities	$	49,924	$	228	$	—	$	50,152
U.S. Agency securities	24,974		86		—		25,060
Mortgage-backed securities:
Government National Mortgage Association	485,689		11,481		(43)		497,127
Federal Home Loan Mortgage Corporation	578,650		18,919		(9)		597,560
Federal National Mortgage Association	287,842		7,788		(16)		295,614
Small Business Assistance Program	244,653		7,884		(58)		252,479
States and political subdivision securities	54,224		1,356		—		55,580
Other	1,006		48		—		1,054
Total	$	1,726,962	$	47,790	$	(126)	$	1,774,626

	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses		Estimated Fair Value
	(dollars in thousands)
As of September 30, 2019
U.S. Treasury securities	$	94,178	$	599	$	(32)	$	94,745
Mortgage-backed securities:
Government National Mortgage Association	501,139		3,374		(3,027)		501,486
Federal Home Loan Mortgage Corporation	463,974		8,840		(770)		472,044
Federal National Mortgage Association	322,340		5,409		(398)		327,351
Small Business Assistance Program	316,502		3,674		(154)		320,022
States and political subdivision securities	66,145		494		(116)		66,523
Other	1,006		31		—		1,037
Total	$	1,765,284	$	22,421	$	(4,497)	$	1,783,208

Summary of amortized cost and approximate fair value of debt securities available for sale

The amortized cost and approximate fair value of debt securities available for sale as of September 30, 2020 and 2019, 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 any penalty.

	September 30, 2020				September 30, 2019
	Amortized  Cost		Estimated Fair Value		Amortized  Cost		Estimated Fair Value
	(dollars in thousands)
Due in one year or less	$	67,131	$	67,456	$	58,377	$	58,343
Due after one year through five years	51,779		52,694		89,836		90,601
Due after five years through ten years	10,212		10,642		12,110		12,324
Due after ten years	—		—		—		—
	129,122		130,792		160,323		161,268
Mortgage-backed securities	1,596,834		1,642,780		1,603,955		1,620,903
Securities without contractual maturities	1,006		1,054		1,006		1,037
Total	$	1,726,962	$	1,774,626	$	1,765,284	$	1,783,208

Summary of unrealized gains and losses on investments

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 September 30, 2020
U.S. Treasury securities	$	—	$	—	$	—	$	—	$	—	$	—
Mortgage-backed securities	71,547		(103)		27,897		(23)		99,444		(126)
States and political subdivision securities	—		—		—		—		—		—
Other	—		—		—		—		—		—
Total	$	71,547	$	(103)	$	27,897	$	(23)	$	99,444	$	(126)

	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, 2019
U.S. Treasury securities	$	—	$	—	$	44,729	$	(32)	$	44,729	$	(32)
Mortgage-backed securities	94,612		(205)		474,979		(4,144)		569,591		(4,349)
States and political subdivision securities	—		—		23,693		(116)		23,693		(116)
Other	—		—		—		—		—		—
Total	$	94,612	$	(205)	$	543,401	$	(4,292)	$	638,013	$	(4,497)