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)
March 31, 2017
Investment securities available for sale:
U.S. Treasury and federal agencies	$	1,907,573		181		9,504	$	1,898,250
Obligations of states and political subdivisions		3,051		77		4		3,124
Mortgage-backed securities:
Government issued or guaranteed		10,525,601		87,830		109,117		10,504,314
Privately issued		42		—		1		41
Other debt securities		133,778		1,300		15,311		119,767
Equity securities		70,199		35,873		568		105,504
		12,640,244		125,261		134,505		12,631,000
Investment securities held to maturity:
Obligations of states and political subdivisions		46,836		233		156		46,913
Mortgage-backed securities:
Government issued or guaranteed		2,670,966		34,777		10,577		2,695,166
Privately issued		153,036		1,191		35,702		118,525
Other debt securities		5,281		—		—		5,281
		2,876,119		36,201		46,435		2,865,885
Other securities		461,296		—		—		461,296
Total	$	15,977,659		161,462		180,940	$	15,958,181

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 quarters ended March 31, 2017 and 2016.

At March 31, 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	$	156,211		156,378
Due after one year through five years		1,756,628		1,747,311
Due after five years through ten years		56,297		54,094
Due after ten years		75,266		63,358
		2,044,402		2,021,141
Mortgage-backed securities available for sale		10,525,643		10,504,355
	$	12,570,045		12,525,496
Debt securities held to maturity:
Due in one year or less	$	21,613		21,711
Due after one year through five years		23,326		23,261
Due after five years through ten years		1,897		1,941
Due after ten years		5,281		5,281
		52,117		52,194
Mortgage-backed securities held to maturity		2,824,002		2,813,691
	$	2,876,119		2,865,885

3. Investment securities, continued

A summary of investment securities that as of March 31, 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)
March 31, 2017
Investment securities available for sale:
U.S. Treasury and federal agencies	$	1,745,277		(9,494	)		2,044		(10	)
Obligations of states and political subdivisions		—		—			472		(4	)
Mortgage-backed securities:
Government issued or guaranteed		5,549,268		(108,380	)		93,425		(737	)
Privately issued		—		—			25		(1	)
Other debt securities		19,052		(100	)		75,763		(15,211	)
Equity securities		17,869		(422	)		154		(146	)
		7,331,466		(118,396	)		171,883		(16,109	)

Investment securities held to maturity:
Obligations of states and political subdivisions		13,205		(87	)		9,909		(69	)
Mortgage-backed securities:
Government issued or guaranteed		896,556		(10,013	)		17,472		(564	)
Privately issued		7,161		(29	)		74,137		(35,673	)
		916,922		(10,129	)		101,518		(36,306	)
Total	$	8,248,388		(128,525	)		273,401		(52,415	)

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 1,075 individual investment securities with aggregate gross unrealized losses of $181 million at March 31, 2017. Based on a review of each of the securities in the investment securities portfolio at March 31, 2017, the Company concluded that it expected to recover the amortized cost basis of its investment. As of March 31, 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 March 31, 2017, 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.