Investment Securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments, Debt and Equity Securities [Abstract]

Investment securities available for sale

December 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale
U.S. Treasury securities	$	1,003	$	—	$	(7	)	$	996
Residential mortgage-related securities
FNMA / FHLMC	457,680		9,722		(2,634		)	464,768
GNMA	1,944,453		275		(31,378		)	1,913,350
Private-label	1,067		—		(8		)	1,059
GNMA commercial mortgage-related securities	1,547,173		5		(33,901		)	1,513,277
FFELP asset backed securities	144,322		867		(13		)	145,176
Other securities (debt and equity)	4,719		127		(26		)	4,820
Total investment securities available for sale	$	4,100,417	$	10,996	$	(67,967	)	$	4,043,446
Investment securities held to maturity
Obligations of state and political subdivisions (municipal securities)	$	1,281,320	$	13,899	$	(3,177	)	$	1,292,042
Residential mortgage-related securities
FNMA / FHLMC	40,995		398		(489		)	40,904
GNMA	414,440		2,700		(6,400		)	410,740
GNMA commercial mortgage-related securities	546,098		9,546		(15,756		)	539,888
Total investment securities held to maturity	$	2,282,853	$	26,543	$	(25,822	)	$	2,283,574

December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale
U.S. Treasury securities	$	1,000	$	—	$	—		$	1,000
Residential mortgage-related securities:
FNMA / FHLMC	625,234		17,298		(2,602		)	639,930
GNMA	2,028,301		1,372		(25,198		)	2,004,475
Private-label	1,134		1		(14		)	1,121
GNMA commercial mortgage-related securities	2,064,508		356		(35,966		)	2,028,898
Other securities (debt and equity)	4,718		105		(21		)	4,802
Total investment securities available for sale	$	4,724,895	$	19,132	$	(63,801	)	$	4,680,226
Investment securities held to maturity
Obligations of state and political subdivisions (municipal securities)	$	1,145,843	$	3,868	$	(12,036	)	$	1,137,675
Residential mortgage-related securities
FNMA / FHLMC	37,697		439		(693		)	37,443
GNMA	89,996		216		(656		)	89,556
Total investment securities held to maturity	$	1,273,536	$	4,523	$	(13,385	)	$	1,264,674

Investment securities held to maturity

December 31, 2017	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale
U.S. Treasury securities	$	1,003	$	—	$	(7	)	$	996
Residential mortgage-related securities
FNMA / FHLMC	457,680		9,722		(2,634		)	464,768
GNMA	1,944,453		275		(31,378		)	1,913,350
Private-label	1,067		—		(8		)	1,059
GNMA commercial mortgage-related securities	1,547,173		5		(33,901		)	1,513,277
FFELP asset backed securities	144,322		867		(13		)	145,176
Other securities (debt and equity)	4,719		127		(26		)	4,820
Total investment securities available for sale	$	4,100,417	$	10,996	$	(67,967	)	$	4,043,446
Investment securities held to maturity
Obligations of state and political subdivisions (municipal securities)	$	1,281,320	$	13,899	$	(3,177	)	$	1,292,042
Residential mortgage-related securities
FNMA / FHLMC	40,995		398		(489		)	40,904
GNMA	414,440		2,700		(6,400		)	410,740
GNMA commercial mortgage-related securities	546,098		9,546		(15,756		)	539,888
Total investment securities held to maturity	$	2,282,853	$	26,543	$	(25,822	)	$	2,283,574

December 31, 2016	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale
U.S. Treasury securities	$	1,000	$	—	$	—		$	1,000
Residential mortgage-related securities:
FNMA / FHLMC	625,234		17,298		(2,602		)	639,930
GNMA	2,028,301		1,372		(25,198		)	2,004,475
Private-label	1,134		1		(14		)	1,121
GNMA commercial mortgage-related securities	2,064,508		356		(35,966		)	2,028,898
Other securities (debt and equity)	4,718		105		(21		)	4,802
Total investment securities available for sale	$	4,724,895	$	19,132	$	(63,801	)	$	4,680,226
Investment securities held to maturity
Obligations of state and political subdivisions (municipal securities)	$	1,145,843	$	3,868	$	(12,036	)	$	1,137,675
Residential mortgage-related securities
FNMA / FHLMC	37,697		439		(693		)	37,443
GNMA	89,996		216		(656		)	89,556
Total investment securities held to maturity	$	1,273,536	$	4,523	$	(13,385	)	$	1,264,674

Amortized cost and fair values of investment securities available for sale by contractual maturity

The expected maturities of investment securities available for sale and held to maturity at December 31, 2017, are shown below. Expected maturities will differ from contractual maturities because borrowers may have the right to call or prepay obligations with or without call or prepayment penalties.

	Available for Sale					Held to Maturity
	Amortized Cost		Fair Value			Amortized Cost		Fair Value
	($ in Thousands)
Due in one year or less	$	1,001	$	1,001		$	50,089	$	44,781
Due after one year through five years	4,703		4,669			226,053		229,939
Due after five years through ten years	—		—			322,413		324,380
Due after ten years	—		—			682,765		692,942
Total debt securities	5,704		5,670			1,281,320		1,292,042
Residential mortgage-related securities
FNMA / FHLMC	457,680		464,768			40,995		40,904
GNMA	1,944,453		1,913,350			414,440		410,740
Private-label	1,067		1,059			—		—
GNMA commercial mortgage-related securities	1,547,173		1,513,277			546,098		539,888
FFELP asset backed securities	144,322		145,176			—		—
Equity securities	18		146			—		—
Total investment securities	$	4,100,417	$	4,043,446		$	2,282,853	$	2,283,574
Ratio of Fair Value to Amortized Cost			98.6		%			100.0		%

Realized gains and losses and proceeds from sale

Total proceeds and gross realized gains and losses from sales of investment securities for each of the three years ended December 31 were as follows. There were no other-than-temporary impairment write-downs on investment securities for 2017, 2016, or 2015.

	2017			2016			2015
	($ in Thousands)
Gross gains on available for sale securities	$	—		$	9,485		$	12,270
Gross gains on held to maturity securities	439			33			—
Total gains	439			9,518			12,270
Gross losses on available for sale securities	—			(202		)	(4,137		)
Gross losses on held to maturity securities	(5		)	—			—
Total losses	(5		)	(202		)	(4,137		)
Investment securities gains, net	$	434		$	9,316		$	8,133
Proceeds from sales of investment securities	$	18,467		$	549,555		$	1,601,947

Unrealized losses and fair value of available for sale and held to maturity securities, by investment category and time length

The following represents gross unrealized losses and the related fair value of investment securities available for sale and held to maturity, aggregated by investment category and length of time individual securities have been in a continuous unrealized loss position, at December 31, 2017.

	Less than 12 months						12 months or more						Total
December 31, 2017	Number of Securities	Unrealized Losses			Fair Value		Number of Securities	Unrealized Losses			Fair Value		Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale
U.S. Treasury securities	1	$	(7	)	$	996	—	$	—		$	—	$	(7	)	$	996
Residential mortgage-related securities
FNMA / FHLMC	9	(572		)	69,939		9	(2,062		)	142,093		(2,634		)	212,032
GNMA	44	(8,927		)	1,028,221		25	(22,451		)	737,198		(31,378		)	1,765,419
Private-label	—	—			—		1	(8		)	1,059		(8		)	1,059
GNMA commercial mortgage-related securities	33	(5,554		)	480,514		70	(28,347		)	1,026,642		(33,901		)	1,507,156
FFELP asset backed securities	1	(13		)	12,158		—	—			—		(13		)	12,158
Other securities (debt and equity)	4	(26		)	1,675		—	—			—		(26		)	1,675
Total	92	$	(15,099	)	$	1,593,503	105	$	(52,868	)	$	1,906,992	$	(67,967	)	$	3,500,495
Investment securities held to maturity
Obligations of state and political subdivisions (municipal securities)	157	$	(746	)	$	122,761	132	$	(2,431	)	$	127,043	$	(3,177	)	$	249,804
Residential mortgage-related securities
FNMA / FHLMC	8	(73		)	13,143		10	(417		)	16,262		(490		)	29,405
GNMA	35	(3,373		)	268,388		18	(3,026		)	120,892		(6,399		)	389,280
GNMA commercial mortgage-related securities	2	(299		)	52,997		23	(15,457		)	486,891		(15,756		)	539,888
Total	202	$	(4,491	)	$	457,289	183	$	(21,331	)	$	751,088	$	(25,822	)	$	1,208,377

For comparative purposes, the following represents gross unrealized losses and the related fair value of investment securities available for sale and held to maturity, aggregated by investment category and length of time that individual securities have been in a continuous unrealized loss position, at December 31, 2016.

	Less than 12 months						12 months or more						Total
December 31, 2016	Number of Securities	Unrealized Losses			Fair Value		Number of Securities	Unrealized Losses			Fair Value		Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale
Residential mortgage-related securities
FNMA / FHLMC	14	$	(2,602	)	$	244,252	—	$	—		$	—	$	(2,602	)	$	244,252
GNMA	54	(25,198		)	1,723,523		—	—			—		(25,198		)	1,723,523
Private-label	—	—			—		1	(14		)	1,119		(14		)	1,119
GNMA commercial mortgage-related securities	74	(16,445		)	1,427,889		21	(19,521		)	429,258		(35,966		)	1,857,147
Other securities (debt and equity)	3	(21		)	1,479		—	—			—		(21		)	1,479
Total	145	$	(44,266	)	$	3,397,143	22	$	(19,535	)	$	430,377	$	(63,801	)	$	3,827,520
Investment securities held to maturity
Obligations of state and political subdivisions (municipal securities)	700	$	(11,937	)	$	414,186	4	$	(99	)	$	1,752	$	(12,036	)	$	415,938
Residential mortgage-related securities
FNMA / FHLMC	14	(441		)	17,477		1	(252		)	6,031		(693		)	23,508
GNMA	39	(656		)	64,633		—	—			—		(656		)	64,633
Total	753	$	(13,034	)	$	496,296	5	$	(351	)	$	7,783	$	(13,385	)	$	504,079