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, 2016
Investment securities available for sale:
U.S. Treasury and federal agencies	$	201,002		1,197		7	$	202,192
Obligations of states and political subdivisions		5,356		138		46		5,448
Mortgage-backed securities:
Government issued or guaranteed		11,490,181		265,879		5,998		11,750,062
Privately issued		65		2		2		65
Collateralized debt obligations		28,483		18,170		1,613		45,040
Other debt securities		136,968		1,407		25,667		112,708
Equity securities		75,271		10,225		364		85,132

		11,937,326		297,018		33,697		12,200,647


Investment securities held to maturity:
Obligations of states and political subdivisions		103,408		886		332		103,962
Mortgage-backed securities:
Government issued or guaranteed		2,445,563		78,448		2,070		2,521,941
Privately issued		175,467		1,848		40,048		137,267
Other debt securities		6,173		—		—		6,173

		2,730,611		81,182		42,450		2,769,343

Other securities		536,062		—		—		536,062

Total	$	15,203,999		378,200		76,147	$	15,506,052


	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

There were no significant gross realized gains or losses from sales of investment securities for the quarters ended March 31, 2016 and 2015.

At March 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	$	7,504		7,551
Due after one year through five years		201,714		203,128
Due after five years through ten years		2,728		2,926
Due after ten years		159,863		151,783

		371,809		365,388
Mortgage-backed securities available for sale		11,490,246		11,750,127

	$	11,862,055		12,115,515


Debt securities held to maturity:
Due in one year or less	$	32,387		32,542
Due after one year through five years		64,484		64,760
Due after five years through ten years		6,537		6,660
Due after ten years		6,173		6,173

		109,581		110,135
Mortgage-backed securities held to maturity		2,621,030		2,659,208

	$	2,730,611		2,769,343

A summary of investment securities that as of March 31, 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)
March 31, 2016
Investment securities available for sale:
U.S. Treasury and federal agencies	$	4,051		(7	)		—		—
Obligations of states and political subdivisions		—		—			1,706		(46	)
Mortgage-backed securities:
Government issued or guaranteed		337,672		(1,959	)		1,233,329		(4,039	)
Privately issued		—		—			34		(2	)
Collateralized debt obligations		10,326		(527	)		1,858		(1,086	)
Other debt securities		9,825		(1,203	)		88,715		(24,464	)
Equity securities		2,115		(210	)		146		(154	)

		363,989		(3,906	)		1,325,788		(29,791	)


Investment securities held to maturity:
Obligations of states and political subdivisions		28,707		(215	)		8,813		(117	)
Mortgage-backed securities:
Government issued or guaranteed		812		(12	)		232,432		(2,058	)
Privately issued		—		—			105,355		(40,048	)

		29,519		(227	)		346,600		(42,223	)

Total	$	393,508		(4,133	)		1,672,388		(72,014	)


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 538 individual investment securities with aggregate gross unrealized losses of $76 million at March 31, 2016. Based on a review of each of the securities in the investment securities portfolio at March 31, 2016, the Company concluded that it expected to recover the amortized cost basis of its investment. As of March 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 March 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 $536 million of cost method investment securities.