Investment Securities (Tables)

12 Months Ended

Dec. 31, 2015

Investments, Debt and Equity Securities [Abstract]

Investment securities available for sale

December 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale:
U. S. Treasury securities	$	999	$	—	$	(2	)	$	997
Residential mortgage-related securities:
FNMA / FHLMC	1,388,995		33,791		(8,160		)	1,414,626
GNMA	1,605,956		507		(16,460		)	1,590,003
Private-label	1,722		1		(14		)	1,709
GNMA commercial mortgage-related securities	1,982,477		1,334		(28,501		)	1,955,310
Other securities (debt and equity)	4,718		51		—			4,769
Total investment securities available for sale	$	4,984,867	$	35,684	$	(53,137	)	$	4,967,414
Investment securities held to maturity:
Obligations of state and political subdivisions (municipal securities)	$	1,043,767	$	16,803	$	(339	)	$	1,060,231
Residential mortgage-related securities:
FNMA / FHLMC	41,469		513		(645		)	41,337
GNMA	82,994		189		(309		)	82,874
Total investment securities held to maturity	$	1,168,230	$	17,505	$	(1,293	)	$	1,184,442

December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale:
U. S. Treasury securities	$	999	$	—	$	(1	)	$	998
Municipal securities	560,839		21,869		(29		)	582,679
Residential mortgage-related securities:
FNMA / FHLMC	3,534,240		59,640		(30,423		)	3,563,457
GNMA	165,863		1,596		(127		)	167,332
Private-label	2,297		7		(10		)	2,294
GNMA commercial mortgage-related securities	1,097,913		1,922		(25,942		)	1,073,893
Other securities (debt and equity)	6,108		51		—			6,159
Total investment securities available for sale	$	5,368,259	$	85,085	$	(56,532	)	$	5,396,812
Investment securities held to maturity:
Municipal securities	$	404,455	$	9,444	$	(832	)	$	413,067
Total investment securities held to maturity	$	404,455	$	9,444	$	(832	)	$	413,067

Investment securities held to maturity

December 31, 2015	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale:
U. S. Treasury securities	$	999	$	—	$	(2	)	$	997
Residential mortgage-related securities:
FNMA / FHLMC	1,388,995		33,791		(8,160		)	1,414,626
GNMA	1,605,956		507		(16,460		)	1,590,003
Private-label	1,722		1		(14		)	1,709
GNMA commercial mortgage-related securities	1,982,477		1,334		(28,501		)	1,955,310
Other securities (debt and equity)	4,718		51		—			4,769
Total investment securities available for sale	$	4,984,867	$	35,684	$	(53,137	)	$	4,967,414
Investment securities held to maturity:
Obligations of state and political subdivisions (municipal securities)	$	1,043,767	$	16,803	$	(339	)	$	1,060,231
Residential mortgage-related securities:
FNMA / FHLMC	41,469		513		(645		)	41,337
GNMA	82,994		189		(309		)	82,874
Total investment securities held to maturity	$	1,168,230	$	17,505	$	(1,293	)	$	1,184,442

December 31, 2014	Amortized Cost		Gross Unrealized Gains		Gross Unrealized Losses			Fair Value
	($ in Thousands)
Investment securities available for sale:
U. S. Treasury securities	$	999	$	—	$	(1	)	$	998
Municipal securities	560,839		21,869		(29		)	582,679
Residential mortgage-related securities:
FNMA / FHLMC	3,534,240		59,640		(30,423		)	3,563,457
GNMA	165,863		1,596		(127		)	167,332
Private-label	2,297		7		(10		)	2,294
GNMA commercial mortgage-related securities	1,097,913		1,922		(25,942		)	1,073,893
Other securities (debt and equity)	6,108		51		—			6,159
Total investment securities available for sale	$	5,368,259	$	85,085	$	(56,532	)	$	5,396,812
Investment securities held to maturity:
Municipal securities	$	404,455	$	9,444	$	(832	)	$	413,067
Total investment securities held to maturity	$	404,455	$	9,444	$	(832	)	$	413,067

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

	Available for Sale					Held to Maturity
($ in Thousands)	Amortized Cost		Fair Value			Amortized Cost		Fair Value
Due in one year or less	$	1,500	$	1,515		$	42,841	$	26,761
Due after one year through five years	4,199		4,197			244,257		255,899
Due after five years through ten years	—		—			253,429		261,934
Due after ten years	—		—			503,240		515,637
Total debt securities	5,699		5,712			1,043,767		1,060,231
Residential mortgage-related securities:
FNMA / FHLMC	1,388,995		1,414,626			41,469		41,337
GNMA	1,605,956		1,590,003			82,994		82,874
Private-label	1,722		1,709			—		—
GNMA commercial mortgage-related securities	1,982,477		1,955,310			—		—
Equity securities	18		54			—		—
Total investment securities	$	4,984,867	$	4,967,414		$	1,168,230	$	1,184,442
Ratio of Fair Value to Amortized Cost			99.6		%			101.4		%

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 2015, 2014, or 2013.

	2015			2014			2013
	($ in Thousands)
Gross gains	$	12,270		$	1,184		$	637
Gross losses	(4,137		)	(690		)	(73		)
Investment securities gains, net	$	8,133		$	494		$	564
Proceeds from sales of investment securities	$	1,601,947		$	102,011		$	136,152

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

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, 2014.

	Less than 12 months						12 months or more						Total
December 31, 2014	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	$	(1	)	$	998	—	$	—		$	—	$	(1	)	$	998
Municipal securities	6	(9		)	3,374		6	(20		)	2,133		(29		)	5,507
Residential mortgage-related securities:
FNMA / FHLMC	15	(1,395		)	330,839		52	(29,028		)	1,246,093		(30,423		)	1,576,932
GNMA	1	(9		)	2,874		4	(118		)	10,440		(127		)	13,314
Private-label	1	(9		)	1,772		2	(1		)	27		(10		)	1,799
GNMA commercial mortgage-related securities	9	(1,766		)	329,982		20	(24,176		)	460,425		(25,942		)	790,407
Total	33	$	(3,189	)	$	669,839	84	$	(53,343	)	$	1,719,118	$	(56,532	)	$	2,388,957
Investment securities held to maturity:
Municipal securities	74	$	(216	)	$	31,924	85	$	(616	)	$	38,915	$	(832	)	$	70,839
Total	74	$	(216	)	$	31,924	85	$	(616	)	$	38,915	$	(832	)	$	70,839

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, 2015.

	Less than 12 months						12 months or more						Total
December 31, 2015	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	$	(2	)	$	997	—	$	—		$	—	$	(2	)	$	997
Residential mortgage-related securities:
FNMA / FHLMC	17	(1,548		)	220,852		14	(6,612		)	338,186		(8,160		)	559,038
GNMA	46	(16,460		)	1,434,484		—	—			—		(16,460		)	1,434,484
Private-label	1	(1		)	83		3	(13		)	1,565		(14		)	1,648
GNMA commercial mortgage-related securities	40	(9,610		)	1,132,844		21	(18,891		)	448,218		(28,501		)	1,581,062
Total	105	$	(27,621	)	$	2,789,260	38	$	(25,516	)	$	787,969	$	(53,137	)	$	3,577,229
Investment securities held to maturity:
Municipal securities	53	$	(146	)	$	23,137	24	$	(193	)	$	9,254	$	(339	)	$	32,391
Residential mortgage-related securities:
FNMA / FHLMC	10	(177		)	12,754		3	(468		)	11,106		(645		)	23,860
GNMA	21	(201		)	45,499		3	(108		)	6,797		(309		)	52,296
Total	84	$	(524	)	$	81,390	30	$	(769	)	$	27,157	$	(1,293	)	$	108,547