The amortized cost and fair values of investment securities available for sale and held to maturity were as follows.

						Gross		Gross
				Amortized		unrealized		unrealized
June 30, 2013:				cost		gains		losses			Fair value
					($ in Thousands)
Investment securities available for sale:
	U.S. Treasury securities			$	1,002	$	2	$	0	$	1,004
	Obligations of state and political subdivisions
		(municipal securities)			701,536		29,903		(1,206)		730,233
	Residential mortgage-related securities:
		Government-sponsored enterprise ("GSE")			3,600,772		64,064		(59,806)		3,605,030
		Private-label			3,403		37		(1)		3,439
	GSE commercial mortgage-related securities				446,807		1,744		(17,299)		431,252
	Other securities (debt and equity)				84,113		746		(1,498)		83,361
			Total investment securities available for sale	$	4,837,633	$	96,496	$	(79,810)	$	4,854,319

Investment securities held to maturity:
	Obligations of state and political subdivisions
		(municipal securities)		$	75,946	$	―	$	(4,448)	$	71,498
			Total investment securities held to maturity	$	75,946	$	―	$	(4,448)	$	71,498

						Gross		Gross
				Amortized		unrealized		unrealized
December 31, 2012:				cost		gains		losses			Fair value
					($ in Thousands)
Investment securities available for sale:
	U.S. Treasury securities			$	1,003	$	1	$	0	$	1,004
	Obligations of state and political subdivisions
		(municipal securities)			755,644		45,599		(55)		801,188
	Residential mortgage-related securities:
		GSE			3,708,287		93,595		(3,727)		3,798,155
		Private-label			6,002		147		0		6,149
	GSE commercial mortgage-related securities				226,420		2,809		(1,063)		228,166
	Other securities (debt and equity)				90,622		1,549		(75)		92,096
			Total investment securities available for sale	$	4,787,978	$	143,700	$	(4,920)	$	4,926,758

Investment securities held to maturity:
	Obligations of state and political subdivisions
		(municipal securities)		$	39,877	$	98	$	(296)	$	39,679
			Total investment securities held to maturity	$	39,877	$	98	$	(296)	$	39,679

The amortized cost and fair values of investment securities available for sale and held to maturity at June 30, 2013, 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
($ in Thousands)		Amortized Cost			Fair Value	Amortized Cost			Fair Value
Due in one year or less		$	51,411	$	51,364	$	-	$	-
Due after one year through five years			218,135		226,577		-		-
Due after five years through ten years			481,965		500,463		26,492		25,262
Due after ten years			35,122		36,149		49,454		46,236
	Total debt securities		786,633		814,553		75,946		71,498
Residential mortgage-related securities:
	GSE		3,600,772		3,605,030		-		-
	Private label		3,403		3,439		-		-
GSE commercial mortgage-related securities			446,807		431,252		-		-
Equity securities			18		45		-		-
	Total investment securities	$	4,837,633	$	4,854,319	$	75,946	$	71,498

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 June 30, 2013.

			Less than 12 months					12 months or more						Total
			Number of	Unrealized			Fair	Number of	Unrealized			Fair	Unrealized			Fair
June 30, 2013:			Securities		Losses		Value	Securities		Losses		Value		Losses		Value
					($ in Thousands)
Investment securities available for sale:
	Obligations of state and political
		subdivisions (municipal securities)	101	$	(1,188)	$	40,506	2	$	(18)	$	511	$	(1,206)	$	41,017
	Residential mortgage-related securities:
		GSE	94		(59,806)		2,011,631	0		―		―		(59,806)		2,011,631
		Private-label	2		(1)		30	1		―		42		(1)		72
	GSE commercial mortgage-related securities		15		(17,299)		410,410	0		―		―		(17,299)		410,410
	Other debt securities		4		(1,453)		5,042	1		(45)		142		(1,498)		5,184
		Total		$	(79,747)	$	2,467,619		$	(63)	$	695	$	(79,810)	$	2,468,314

Investment securities held to maturity:
	Obligations of state and political
		subdivisions (municipal securities)	160	$	(4,448)	$	69,951	―	$	―	$	―	$	(4,448)	$	69,951
		Total		$	(4,448)	$	69,951		$	―	$	―	$	(4,448)	$	69,951

The Corporation reviews the investment securities portfolio on a quarterly basis to monitor its exposure to other-than-temporary impairment. A determination as to whether a security's decline in fair value is other-than-temporary takes into consideration numerous factors and the relative significance of any single factor can vary by security. Some factors the Corporation may consider in the other-than-temporary impairment analysis include, the length of time and extent to which the security has been in an unrealized loss position, changes in security ratings, financial condition and near-term prospects of the issuer, as well as security and industry specific economic conditions. In addition, with regards to its debt securities, the Corporation may also evaluate payment structure, whether there are defaulted payments or expected defaults, prepayment speeds, and the value of any underlying collateral. For certain debt securities in unrealized loss positions, the Corporation prepares cash flow analyses to compare the present value of cash flows expected to be collected from the security with the amortized cost basis of the security.

Based on the Corporation's evaluation, management does not believe any unrealized loss at June 30, 2013 represents an other-than-temporary impairment as these unrealized losses are primarily attributable to changes in interest rates and the current market conditions, and not credit deterioration. The unrealized losses reported for residential mortgage-related securities relate to non-agency residential mortgage-related securities as well as residential mortgage-related securities issued by government agencies such as the Federal National Mortgage Association (“FNMA”) and the Federal Home Loan Mortgage Corporation (“FHLMC”). The unrealized losses reported for commercial mortgage-related securities relate to government agency mortgage-related securities. At June 30, 2013, the unrealized loss position of 12 months or more on other debt securities was attributable to a pooled trust preferred debt security. The Corporation currently does not intend to sell nor does it believe that it will be required to sell the securities contained in the above unrealized losses table before recovery of their amortized cost basis.

The following is a summary of the credit loss portion of other-than-temporary impairment recognized in earnings on debt securities for 2012 and the six months ended June 30, 2013, respectively.

		Non-agency
		Mortgage-Related		Trust Preferred
		Securities		Debt Securities			Total
		($ in Thousands)
Balance of credit-related other-than-temporary
	impairment at December 31, 2011	$	(17,558)	$	(10,835)	$	(28,393)
Reduction due to credit impaired securities sold			17,026		4,499		21,525
Balance of credit-related other-than-temporary							.
	impairment at December 31, 2012	$	(532)	$	(6,336)	$	(6,868)
Reduction due to credit impaired securities sold			532		―		532
Balance of credit-related other-than-temporary							.
	impairment at June 30, 2013	$	―	$	(6,336)	$	(6,336)

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, 2012.

					Less than 12 months					12 months or more				Total
			Number					Number
			of	Unrealized				of	Unrealized				Unrealized
December 31, 2012:			Securities	Losses		Fair Value		Securities	Losses		Fair Value		Losses		Fair Value
					($ in Thousands)
Investment securities available for sale:
	Obligations of state and political
		subdivisions (municipal securities)	15	$	(42)	$	5,065	1	$	(13)	$	348	$	(55)	$	5,413
	Residential mortgage-related securities:
		GSE	30		(3,727)		892,964	0		-		-		(3,727)		892,964
		Private label	0		-		-	1		-		50		-		50
	GSE commercial mortgage-related securities		2		(1,063)		102,474	0		-		-		(1,063)		102,474
	Other debt securities		0		-		-	1		(75)		111		(75)		111
		Total		$	(4,832)	$	1,000,503		$	(88)	$	509	$	(4,920)	$	1,001,012

Investment securities held to maturity:
	Obligations of state and political
		subdivisions (municipal securities)	56	$	(296)	$	28,265	0	$	-	$	-	$	(296)	$	28,265
		Total		$	(296)	$	28,265		$	-	$	-	$	(296)	$	28,265

Federal Home Loan Bank (“FHLB”) and Federal Reserve Bank Stocks: The Corporation is required to maintain Federal Reserve stock and FHLB stock as a member of both the Federal Reserve System and the FHLB, and in amounts as required by these institutions. These equity securities are “restricted” in that they can only be sold back to the respective institutions or another member institution at par. Therefore, they are less liquid than other marketable equity securities and their fair value is equal to amortized cost. At June 30, 2013 and December 31, 2012, the Corporation had FHLB stock of $110 million and $96 million, respectively. The Corporation had Federal Reserve Bank stock of $71 million at both June 30, 2013 and December 31, 2012.

The Corporation reviewed these securities for impairment, including but not limited to, consideration of operating performance, the severity and duration of market value declines, as well as its liquidity and funding position. After evaluating all of these considerations, the Corporation believes the cost of these investments will be recovered and no impairment has been recorded on these securities during 2012 or the first six months of 2013. The FHLB of Chicago initiated tender offers for certain of its shares during the first quarter of 2013, whereby the FHLB would repurchase its shares at par. The Corporation participated in the tender offers and reduced its equity holdings in the FHLB of Chicago by $14 million during the first quarter of 2013. Based on the Corporation's periodic review of FHLB stock required member holdings (which is based on asset size and other criteria), the Corporation purchased $28 million of additional equity holdings in the FHLB of Chicago during the second quarter of 2013.