Investment Securities (Tables)

12 Months Ended

Dec. 31, 2022

Investments, Debt and Equity Securities [Abstract]

Summary of Amortized Cost and Estimated Fair Value of Investment Securities

The amortized cost and estimated fair value of investment securities are summarized below. The majority of securities held are available-for-sale securities with fair value based on quoted prices for similar assets in active markets or quoted prices for identical assets in markets that are not active. Estimated fair values were obtained from an independent pricing service based upon market quotes.

	December 31, 2022
	Amortized Cost		Gross Unrealized Holding Gain		Gross Unrealized Holding Loss			Fair Value		Total Percent
	(Dollars in thousands)
Investment securities available-for-sale:
Mortgage-backed securities	$	3,192,151	$	39	$	(403,049	)	$	2,789,141		85.68	%
CMO/REMIC		535,269		—		(95,966	)		439,303		13.50	%
Municipal bonds		26,797		67		(1,177	)		25,687		0.79	%
Other securities		1,080		—		—			1,080		0.03	%
Total available-for-sale securities	$	3,755,297	$	106	$	(500,192	)	$	3,255,211		100.00	%
Investment securities held-to-maturity:
Government agency/GSE	$	548,771	$	—	$	(114,343	)	$	434,428		21.48	%
Mortgage-backed securities		706,796		—		(105,867	)		600,929		27.67	%
CMO/REMIC		827,346		—		(131,730	)		695,616		32.39	%
Municipal bonds		471,388		913		(47,687	)		424,614		18.46	%
Total held-to-maturity securities	$	2,554,301	$	913	$	(399,627	)	$	2,155,587		100.00	%

	December 31, 2021
	Amortized Cost		Gross Unrealized Holding Gain		Gross Unrealized Holding Loss			Fair Value		Total Percent
	(Dollars in thousands)
Investment securities available-for-sale:
Mortgage-backed securities	$	2,553,246	$	25,873	$	(15,905	)	$	2,563,214		80.50	%
CMO/REMIC		602,555		1,586		(13,983	)		590,158		18.53	%
Municipal bonds		28,365		1,103		—			29,468		0.93	%
Other securities		1,083		—		—			1,083		0.04	%
Total available-for-sale securities	$	3,185,249	$	28,562	$	(29,888	)	$	3,183,923		100.00	%
Investment securities held-to-maturity:
Government agency/GSE	$	576,899	$	5,907	$	(7,312	)	$	575,494		29.95	%
Mortgage-backed securities		647,390		4,109		(6,106	)		645,393		33.61	%
CMO/REMIC		490,670		596		(5,030	)		486,236		25.48	%
Municipal bonds		211,011		4,714		(1,155	)		214,570		10.96	%
Total held-to-maturity securities	$	1,925,970	$	15,326	$	(19,603	)	$	1,921,693		100.00	%

Summary of Interest Income Earned on Investment Securities

The following table provides information about the amount of interest income earned on investment securities which is fully taxable and which is exempt from regular federal income tax.

	Year Ended December 31,
	2022		2021		2020
	(Dollars in thousands)
Investment securities available-for-sale:
Taxable	$	67,803	$	37,532	$	35,129
Tax-advantaged		705		741		923
Total interest income from available-for-sale securities		68,508		38,273		36,052
Investment securities held-to-maturity:
Taxable		41,403		17,747		9,542
Tax-advantaged		7,645		4,428		4,681
Total interest income from held-to-maturity securities		49,048		22,175		14,223
Total interest income from investment securities	$	117,556	$	60,448	$	50,275

Summary of Continuous Unrealized Loss Position of Securities

The following table presents the Company’s available-for-sale investment securities, by investment category, in an unrealized loss position for which an allowance for credit losses has not been recorded as of December 31, 2022.

	December 31, 2022
	Less Than 12 Months					12 Months or Longer					Total
	Fair Value		Gross Unrealized Holding Losses			Fair Value		Gross Unrealized Holding Losses			Fair Value		Gross Unrealized Holding Losses
	(Dollars in thousands)
Investment securities available-for-sale:
Mortgage-backed securities	$	1,658,331	$	(187,842	)	$	1,129,257	$	(215,207	)	$	2,787,588	$	(403,049	)
CMO/REMIC		54,005		(4,796	)		385,295		(91,170	)		439,300		(95,966	)
Municipal bonds		24,507		(1,177	)		—		—			24,507		(1,177	)
Total available-for-sale securities	$	1,736,843	$	(193,815	)	$	1,514,552	$	(306,377	)	$	3,251,395	$	(500,192	)
Investment securities held-to-maturity:
Government agency/GSE	$	179,348	$	(39,866	)	$	255,080	$	(74,477	)	$	434,428	$	(114,343	)
Mortgage-backed securities		188,480		(9,042	)		412,449		(96,825	)		600,929		(105,867	)
CMO/REMIC		376,540		(60,598	)		319,076		(71,132	)		695,616		(131,730	)
Municipal bonds		312,702		(35,656	)		53,350		(12,031	)		366,052		(47,687	)
Total held-to-maturity securities	$	1,057,070	$	(145,162	)	$	1,039,955	$	(254,465	)	$	2,097,025	$	(399,627	)

The following table presents the Company’s available-for-sale investment securities, by investment category, in an unrealized loss position for which an allowance for credit losses has not been recorded as of December 31, 2021.

	December 31, 2021
	Less Than 12 Months					12 Months or Longer					Total
	Fair Value		Gross Unrealized Holding Losses			Fair Value		Gross Unrealized Holding Losses			Fair Value		Gross Unrealized Holding Losses
	(Dollars in thousands)
Investment securities available-for-sale:
Mortgage-backed securities	$	1,465,647	$	(15,099	)	$	44,244	$	(806	)	$	1,509,891	$	(15,905	)
CMO/REMIC		450,393		(11,515	)		53,745		(2,468	)		504,138		(13,983	)
Municipal bonds		—		—			—		—			—		—
Total available-for-sale securities	$	1,916,040	$	(26,614	)	$	97,989	$	(3,274	)	$	2,014,029	$	(29,888	)

Summary of Amortized Cost and Fair Value of Debt Securities by Contractual Maturity

Mortgage-backed and CMO/REMIC securities are included in maturity categories based upon estimated average lives which incorporate estimated prepayment speeds.

	December 31, 2022
	Available-for-sale				Held-to-maturity
	Amortized Cost		Fair Value		Amortized Cost		Fair Value
			(Dollars in thousands)
Due in one year or less	$	1,578	$	1,583	$	2,000	$	1,995
Due after one year through five years		345,663		330,565		41,895		40,245
Due after five years through ten years		1,643,898		1,420,508		263,552		234,866
Due after ten years		1,764,158		1,502,555		2,246,854		1,878,481
Total investment securities	$	3,755,297	$	3,255,211	$	2,554,301	$	2,155,587