3. Investment securities

The amortized cost and estimated fair value of investment securities were as follows:


	Amortized cost		Gross unrealized gains		Gross unrealized losses		Estimated fair value
	(in thousands)
September 30, 2015
Investment securities available for sale:
U.S. Treasury and federal agencies	$	197,764		1,630		—	$	199,394
Obligations of states and political subdivisions		6,174		170		48		6,296
Mortgage-backed securities:
Government issued or guaranteed		10,504,756		228,333		19,041		10,714,048
Privately issued		82		2		2		82
Collateralized debt obligations		28,467		22,465		1,056		49,876
Other debt securities		136,793		1,650		17,975		120,468
Equity securities		49,198		20,360		213		69,345

		10,923,234		274,610		38,335		11,159,509

Investment securities held to maturity:
Obligations of states and political subdivisions		125,251		1,395		347		126,299
Mortgage-backed securities:
Government issued or guaranteed		2,679,546		69,486		4,787		2,744,245
Privately issued		186,883		1,628		40,174		148,337
Other debt securities		6,806		—		—		6,806

		2,998,486		72,509		45,308		3,025,687

Other securities		336,544		—		—		336,544

Total	$	14,258,264		347,119		83,643	$	14,521,740


	Amortized cost		Gross unrealized gains		Gross unrealized losses		Estimated fair value
	(in thousands)
December 31, 2014
Investment securities available for sale:
U.S. Treasury and federal agencies	$	161,408		544		5	$	161,947
Obligations of states and political subdivisions		8,027		224		53		8,198
Mortgage-backed securities:
Government issued or guaranteed		8,507,571		223,889		337		8,731,123
Privately issued		104		2		3		103
Collateralized debt obligations		30,073		21,276		1,033		50,316
Other debt securities		138,240		1,896		18,648		121,488
Equity securities		73,901		11,020		1,164		83,757

		8,919,324		258,851		21,243		9,156,932

Investment securities held to maturity:
Obligations of states and political subdivisions		148,961		2,551		189		151,323
Mortgage-backed securities:
Government issued or guaranteed		3,149,320		78,485		7,000		3,220,805
Privately issued		201,733		1,143		44,576		158,300
Other debt securities		7,854		—		—		7,854

		3,507,868		82,179		51,765		3,538,282

Other securities		328,742		—		—		328,742

Total	$	12,755,934		341,030		73,008	$	13,023,956

There were no significant gross realized gains or losses from the sale of investment securities for the three-month and nine-month periods ended September 30, 2015 and 2014, respectively.

At September 30, 2015, the amortized cost and estimated fair value of debt securities by contractual maturity were as follows:


	Amortized cost		Estimated fair value
	(in thousands)
Debt securities available for sale:
Due in one year or less	$	8,282		8,330
Due after one year through five years		197,796		199,712
Due after five years through ten years		3,296		3,495
Due after ten years		159,824		164,497

		369,198		376,034
Mortgage-backed securities available for sale		10,504,838		10,714,130

	$	10,874,036		11,090,164

Debt securities held to maturity:
Due in one year or less	$	30,523		30,706
Due after one year through five years		74,511		75,150
Due after five years through ten years		20,217		20,443
Due after ten years		6,806		6,806

		132,057		133,105
Mortgage-backed securities held to maturity		2,866,429		2,892,582

	$	2,998,486		3,025,687

A summary of investment securities that as of September 30, 2015 and December 31, 2014 had been in a continuous unrealized loss position for less than twelve months and those that had been in a continuous unrealized loss position for twelve months or longer follows:


	Less than 12 months					12 months or more
	Fair value		Unrealized losses			Fair value		Unrealized losses
	(in thousands)
September 30, 2015
Investment securities available for sale:
Obligations of states and political subdivisions	$	516		(2	)		1,405		(46	)
Mortgage-backed securities:
Government issued or guaranteed		1,942,135		(18,925	)		5,503		(116	)
Privately issued		—		—			48		(2	)
Collateralized debt obligations		5,733		(324	)		2,155		(732	)
Other debt securities		19,335		(458	)		86,813		(17,517	)
Equity securities		—		—			212		(213	)

		1,967,719		(19,709	)		96,136		(18,626	)

Investment securities held to maturity:
Obligations of states and political subdivisions		37,336		(292	)		4,096		(55	)
Mortgage-backed securities:
Government issued or guaranteed		16,619		(129	)		240,730		(4,658	)
Privately issued		—		—			118,840		(40,174	)

		53,955		(421	)		363,666		(44,887	)

Total	$	2,021,674		(20,130	)		459,802		(63,513	)

December 31, 2014
Investment securities available for sale:
U.S. Treasury and federal agencies	$	6,505		(5	)		—		—
Obligations of states and political subdivisions		1,785		(52	)		121		(1	)
Mortgage-backed securities:
Government issued or guaranteed		39,001		(186	)		5,555		(151	)
Privately issued		—		—			65		(3	)
Collateralized debt obligations		2,108		(696	)		5,512		(337	)
Other debt securities		14,017		(556	)		92,661		(18,092	)
Equity securities		2,138		(1,164	)		—		—

		65,554		(2,659	)		103,914		(18,584	)

Investment securities held to maturity:
Obligations of states and political subdivisions		29,886		(184	)		268		(5	)
Mortgage-backed securities:
Government issued or guaranteed		137,413		(361	)		446,780		(6,639	)
Privately issued		—		—			127,512		(44,576	)

		167,299		(545	)		574,560		(51,220	)

Total	$	232,853		(3,204	)		678,474		(69,804	)

The Company owned 320 individual investment securities with aggregate gross unrealized losses of $84 million at September 30, 2015. Based on a review of each of the securities in the investment securities portfolio at September 30, 2015, the Company concluded that it expected to recover the amortized cost basis of its investment. As of September 30, 2015, the Company does not intend to sell nor is it anticipated that it would be required to sell any of its impaired investment securities at a loss. At September 30, 2015, the Company has not identified events or changes in circumstances which may have a significant adverse effect on the fair value of the $337 million of cost method investment securities.