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, 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

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

No investment in securities of a single non-U.S. Government or government agency issuer exceeded ten percent of shareholders’ equity at December 31, 2015.

As of December 31, 2015, the latest available investment ratings of all obligations of states and political subdivisions, privately issued mortgage-backed securities, collateralized debt obligations and other debt securities were:


	Amortized Cost		Estimated Fair Value		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	$	124,355	$	125,041	$	95,246	$	—	$	—	$	—	$	29,795
Privately issued mortgage-backed securities		181,165		141,902		39,020		17		—		102,821		44
Collateralized debt obligations		28,438		47,393		9,629		1,485		1,175		35,104		—
Other debt securities		144,131		125,455		8,411		61,379		29,354		18,600		7,711

Total	$	478,089	$	439,791	$	152,306	$	62,881	$	30,529	$	156,525	$	37,550

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


	December 31
	2015		2014
	(In thousands)
Collateralized mortgage obligations:
Amortized cost	$	188,819	$	209,107
Estimated fair value		149,632		165,860

Gross realized gains from sales of investment securities were $116,490,000 in 2013. During 2013, the Company sold its holdings of Visa Class B shares for a gain of $89,545,000 and its holdings of MasterCard Class B shares for a gain of $13,208,000. Gross realized losses on investment securities were $60,033,000 in 2013. The Company sold substantially all of its privately issued mortgage-backed securities held in the available-for-sale investment securities portfolio during 2013. In total, $1.0 billion of such securities were sold for a net loss of approximately $46,302,000. There were no significant gross realized gains or losses from the sale of investment securities in 2015 or 2014.

The Company recognized $10 million of pre-tax other-than-temporary impairment losses related to privately issued mortgage-backed securities in 2013. The impairment charges were recognized in light of deterioration of real estate values and a rise in delinquencies and charge-offs of underlying mortgage loans collateralizing those securities. The other-than-temporary impairment losses represented management’s estimate of credit losses inherent in the debt securities considering projected cash flows using assumptions for delinquency rates, loss severities, and other estimates of future collateral performance. There were no other-than-temporary impairment losses in 2015 or 2014.

At December 31, 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	$	106,349	$	106,383
Due after one year through five years		202,864		203,235
Due after five years through ten years		2,729		2,941
Due after ten years		159,866		159,739

		471,808		472,298
Mortgage-backed securities available for sale		11,593,033		11,686,702

	$	12,064,841	$	12,159,000

Debt securities held to maturity:
Due in one year or less	$	34,174	$	34,311
Due after one year through five years		72,120		72,419
Due after five years through ten years		12,137		12,283
Due after ten years		6,575		6,575

		125,006		125,588
Mortgage-backed securities held to maturity		2,734,703		2,738,559

	$	2,859,709	$	2,864,147

A summary of investment securities that as of December 31, 2015 and 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)
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	)

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 1,007 individual investment securities with aggregate gross unrealized losses of $120 million at December 31, 2015. Based on a review of each of the securities in the investment securities portfolio at December 31, 2015, the Company concluded that it expected to recover the amortized cost basis of its investment. As of December 31, 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 December 31, 2015, the Company has not identified events or changes in circumstances which may have a significant adverse effect on the fair value of the $554 million of cost method investment securities.

At December 31, 2015, investment securities with a carrying value of $4,245,300,000, including $3,333,653,000 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 $2,136,712,000 at December 31, 2015. The pledged securities included securities of the U.S. Treasury and federal agencies and mortgage-backed securities.