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)
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

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

No investment in securities of a single non-U.S. Government, government agency or government guaranteed issuer exceeded ten percent of shareholders’ equity at December 31, 2016.

As of December 31, 2016, the latest available investment ratings of all obligations of states and political subdivisions, privately issued mortgage-backed securities and other debt securities were:

					Average Credit Rating of Fair Value Amount
	Amortized Cost		Estimated Fair Value		A or Better		BBB		BB		B or Less		Not Rated
	(In thousands)

Obligations of states and political subdivisions	$	64,428	$	64,542	$	47,023	$	—	$	—	$	—	$	17,519
Privately issued mortgage-backed securities		157,749		121,525		30,760		16		—		90,730		19
Other debt securities		139,648		124,059		5,442		63,353		30,373		—		24,891
Total	$	361,825	$	310,126	$	83,225	$	63,369	$	30,373	$	90,730	$	42,429

The amortized cost and estimated fair value of collateralized mortgage obligations included in mortgage-backed securities were as follows:

	December 31
	2016		2015
	(In thousands)
Collateralized mortgage obligations:
Amortized cost	$	162,027	$	188,819
Estimated fair value		125,848		149,632

Gross realized gains from sales of investment securities were $30,545,000 in 2016. During 2016, the Company sold its collateralized debt obligations held in the available-for-sale investment securities portfolio for a gain of $30 million. There were no significant realized gross losses from sales of investments securities in 2016. There were no significant gross realized gains or losses from sales of investment securities in 2015 or 2014.

At December 31, 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	$	157,954	$	158,334
Due after one year through five years		1,760,301		1,750,542
Due after five years through ten years		2,689		3,132
Due after ten years		128,841		112,693
		2,049,785		2,024,701
Mortgage-backed securities available for sale		10,980,552		10,954,905
	$	13,030,337	$	12,979,606
Debt securities held to maturity:
Due in one year or less	$	24,533	$	24,643
Due after one year through five years		34,073		33,963
Due after five years through ten years		2,252		2,295
Due after ten years		5,543		5,543
		66,401		66,444
Mortgage-backed securities held to maturity		2,390,877		2,384,778
	$	2,457,278	$	2,451,222

A summary of investment securities that as of December 31, 2016 and 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)
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	)

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 1,083 individual investment securities with aggregate gross unrealized losses of $186 million at December 31, 2016. Based on a review of each of the securities in the investment securities portfolio at December 31, 2016, the Company concluded that it expected to recover the amortized cost basis of its investment. As of December 31, 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 December 31, 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.

At December 31, 2016, investment securities with a carrying value of $3,775,571,000, including $3,240,079,000 of investment securities available for sale, were pledged to secure borrowings from various FHLBs, repurchase agreements, governmental deposits, interest rate swap agreements and available lines of credit as described in note 9.

Investment securities pledged by the Company to secure obligations whereby the secured party is permitted by contract or custom to sell or repledge such collateral totaled $1,203,473,000 at December 31, 2016. The pledged securities included securities of the U.S. Treasury and federal agencies and mortgage-backed securities.