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)
June 30, 2017
Investment securities available for sale:
U.S. Treasury and federal agencies	$	2,118,665		39		10,838	$	2,107,866
Obligations of states and political subdivisions		2,849		66		2		2,913
Mortgage-backed securities:
Government issued or guaranteed		9,601,810		84,797		78,425		9,608,182
Privately issued		36		—		1		35
Other debt securities		133,959		2,923		11,059		125,823
Equity securities		53,832		30,680		466		84,046
		11,911,151		118,505		100,791		11,928,865
Investment securities held to maturity:
Obligations of states and political subdivisions		38,958		211		125		39,044
Mortgage-backed securities:
Government issued or guaranteed		3,196,908		35,630		11,886		3,220,652
Privately issued		147,189		1,706		33,272		115,623
Other debt securities		5,213		—		—		5,213
		3,388,268		37,547		45,283		3,380,532
Other securities		498,927		—		—		498,927
Total	$	15,798,346		156,052		146,074	$	15,808,324

December 31, 2016
Investment securities available for sale:
U.S. Treasury and federal agencies	$	1,912,110	386	9,952	$	1,902,544
Obligations of states and political subdivisions		3,570	77	6		3,641
Mortgage-backed securities:
Government issued or guaranteed		10,980,507	88,343	113,989		10,954,861
Privately issued		45	—	1		44
Other debt securities		134,105	1,407	16,996		118,516
Equity securities		307,964	45,073	571		352,466
		13,338,301	135,286	141,515		13,332,072
Investment securities held to maturity:
Obligations of states and political subdivisions		60,858	267	224		60,901
Mortgage-backed securities:
Government issued or guaranteed		2,233,173	37,498	7,374		2,263,297
Privately issued		157,704	897	37,120		121,481
Other debt securities		5,543	—	—		5,543
		2,457,278	38,662	44,718		2,451,222
Other securities		461,118	—	—		461,118
Total	$	16,256,697	173,948	186,233	$	16,244,412

3. Investment securities, continued

There were no significant gross realized gains or losses from sales of investment securities for the three-month and six-month periods ended June 30, 2017 and 2016, respectively.

At June 30, 2017, 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	$	393,523		392,835
Due after one year through five years		1,730,289		1,720,332
Due after five years through ten years		72,374		72,728
Due after ten years		59,287		50,707
		2,255,473		2,236,602
Mortgage-backed securities available for sale		9,601,846		9,608,217
	$	11,857,319		11,844,819
Debt securities held to maturity:
Due in one year or less	$	21,284		21,363
Due after one year through five years		16,110		16,068
Due after five years through ten years		1,564		1,613
Due after ten years		5,213		5,213
		44,171		44,257
Mortgage-backed securities held to maturity		3,344,097		3,336,275
	$	3,388,268		3,380,532

3. Investment securities, continued

A summary of investment securities that as of June 30, 2017 and December 31, 2016 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)
June 30, 2017
Investment securities available for sale:
U.S. Treasury and federal agencies	$	1,952,450		(10,838	)		—		—
Obligations of states and political subdivisions		—		—			474		(2	)
Mortgage-backed securities:
Government issued or guaranteed		4,560,342		(77,584	)		89,307		(841	)
Privately issued		—		—			19		(1	)
Other debt securities		998		(2	)		60,422		(11,057	)
Equity securities		18,066		(320	)		154		(146	)
		6,531,856		(88,744	)		150,376		(12,047	)

Investment securities held to maturity:
Obligations of states and political subdivisions		8,698		(68	)		8,496		(57	)
Mortgage-backed securities:
Government issued or guaranteed		1,046,740		(11,324	)		18,420		(562	)
Privately issued		—		—			58,671		(33,272	)
		1,055,438		(11,392	)		85,587		(33,891	)
Total	$	7,587,294		(100,136	)		235,963		(45,938	)

December 31, 2016
Investment securities available for sale:
U.S. Treasury and federal agencies	$	1,710,241		(9,950	)		2,295		(2	)
Obligations of states and political subdivisions		—		—			593		(6	)
Mortgage-backed securities:
Government issued or guaranteed		6,730,829		(113,374	)		81,003		(615	)
Privately issued		—		—			27		(1	)
Other debt securities		100		(1	)		85,400		(16,995	)
Equity securities		17,776		(422	)		151		(149	)
		8,458,946		(123,747	)		169,469		(17,768	)

Investment securities held to maturity:
Obligations of states and political subdivisions		17,988		(126	)		11,891		(98	)
Mortgage-backed securities:
Government issued or guaranteed		618,832		(6,842	)		17,481		(532	)
Privately issued		17,911		(1,222	)		57,016		(35,898	)
		654,731		(8,190	)		86,388		(36,528	)
Total	$	9,113,677		(131,937	)		255,857		(54,296	)

The Company owned 1,038 individual investment securities with aggregate gross unrealized losses of $146 million at June 30, 2017. Based on a review of each of the securities in the investment securities portfolio at June 30, 2017, the Company concluded that it expected to recover the amortized cost basis of its investment. As of June 30, 2017, the Company does not intend to sell nor is it anticipated that it would be required to sell any of its impaired

3. Investment securities, continued

investment securities at a loss. At June 30, 2017, the Company has not identified events or changes in circumstances which may have a significant adverse effect on the carrying value of the $499 million of cost method investment securities.