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, 2016
Investment securities available for sale:
U.S. Treasury and federal agencies	$	724,540		1,864		3	$	726,401
Obligations of states and political subdivisions		4,912		108		5		5,015
Mortgage-backed securities:
Government issued or guaranteed		10,617,605		314,088		1,031		10,930,662
Privately issued		52		—		1		51
Other debt securities		134,393		1,802		17,969		118,226
Equity securities		70,112		12,363		263		82,212
		11,551,614		330,225		19,272		11,862,567
Investment securities held to maturity:
Obligations of states and political subdivisions		72,258		458		220		72,496
Mortgage-backed securities:
Government issued or guaranteed		2,168,460		83,630		215		2,251,875
Privately issued		163,489		1,616		39,122		125,983
Other debt securities		5,743		—		—		5,743
		2,409,950		85,704		39,557		2,456,097
Other securities		461,057		—		—		461,057
Total	$	14,422,621		415,929		58,829	$	14,779,721

December 31, 2015
Investment securities available for sale:
U.S. Treasury and federal agencies	$	299,890	294	187	$	299,997
Obligations of states and political subdivisions		5,924	146	42		6,028
Mortgage-backed securities:
Government issued or guaranteed		11,592,959	142,370	48,701		11,686,628
Privately issued		74	2	2		74
Collateralized debt obligations		28,438	20,143	1,188		47,393
Other debt securities		137,556	1,514	20,190		118,880
Equity securities		73,795	10,230	354		83,671
		12,138,636	174,699	70,664		12,242,671
Investment securities held to maturity:
Obligations of states and political subdivisions		118,431	1,003	421		119,013
Mortgage-backed securities:
Government issued or guaranteed		2,553,612	50,936	7,817		2,596,731
Privately issued		181,091	2,104	41,367		141,828
Other debt securities		6,575	—	—		6,575
		2,859,709	54,043	49,605		2,864,147
Other securities		554,059	—	—		554,059
Total	$	15,552,404	228,742	120,269	$	15,660,877

3. Investment securities, continued

During the three months ended September 30, 2016, the Company sold substantially all of its collateralized debt obligations held in the available-for-sale investment securities portfolio for a gain of $28 million. There were no other significant gross realized gains or losses from sales of investment securities for the three-month and nine-month periods ended September 30, 2016 and 2015.

At September 30, 2016, 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	$	105,225		105,679
Due after one year through five years		625,911		627,585
Due after five years through ten years		3,968		4,538
Due after ten years		128,741		111,840
		863,845		849,642
Mortgage-backed securities available for sale		10,617,657		10,930,713
	$	11,481,502		11,780,355
Debt securities held to maturity:
Due in one year or less	$	25,141		25,301
Due after one year through five years		43,474		43,474
Due after five years through ten years		3,643		3,721
Due after ten years		5,743		5,743
		78,001		78,239
Mortgage-backed securities held to maturity		2,331,949		2,377,858
	$	2,409,950		2,456,097

3. Investment securities, continued

A summary of investment securities that as of September 30, 2016 and December 31, 2015 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, 2016
Investment securities available for sale:
U.S. Treasury and federal agencies	$	3,355		(3	)		—		—
Obligations of states and political subdivisions		—		—			472		(5	)
Mortgage-backed securities:
Government issued or guaranteed		224,128		(904	)		7,294		(127	)
Privately issued		—		—			34		(1	)
Other debt securities		—		—			106,876		(17,969	)
Equity securities		767		(101	)		138		(162	)
		228,250		(1,008	)		114,814		(18,264	)

Investment securities held to maturity:
Obligations of states and political subdivisions		19,557		(113	)		11,050		(107	)
Mortgage-backed securities:
Government issued or guaranteed		74,629		(46	)		19,256		(169	)
Privately issued		17,542		(2,371	)		91,127		(36,751	)
		111,728		(2,530	)		121,433		(37,027	)
Total	$	339,978		(3,538	)		236,247		(55,291	)

December 31, 2015
Investment securities available for sale:
U.S. Treasury and federal agencies	$	147,508		(187	)		—		—
Obligations of states and political subdivisions		865		(2	)		1,335		(40	)
Mortgage-backed securities:
Government issued or guaranteed		4,061,899		(48,534	)		7,216		(167	)
Privately issued		—		—			43		(2	)
Collateralized debt obligations		5,711		(335	)		2,063		(853	)
Other debt securities		12,935		(462	)		93,344		(19,728	)
Equity securities		18,073		(207	)		153		(147	)
		4,246,991		(49,727	)		104,154		(20,937	)

Investment securities held to maturity:
Obligations of states and political subdivisions	42,913		(335)			5,853		(86)
Mortgage-backed securities:
Government issued or guaranteed		459,983		(1,801	)		228,867		(6,016	)
Privately issued		—		—			112,155		(41,367	)
		502,896		(2,136	)		346,875		(47,469	)
Total	$	4,749,887		(51,863	)		451,029		(68,406	)

The Company owned 504 individual investment securities with aggregate gross unrealized losses of $59 million at September 30, 2016. Based on a review of each of the securities in the investment securities portfolio at September 30, 2016, the Company concluded that it expected to recover the amortized cost basis of its investment. As of September 30, 2016, 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, 2016, the Company has not identified events or changes in circumstances which may have a significant adverse effect on the fair value of the $461 million of cost method investment securities.