Investment securities (Tables)

12 Months Ended

Dec. 31, 2017

Investments Debt And Equity Securities [Abstract]

Amortized Cost and Estimated Fair Value of 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, 2017
Investment securities available for sale:
U.S. Treasury and federal agencies	$	1,965,665	$	—	$	18,178	$	1,947,487
Obligations of states and political subdivisions		2,555		36		2		2,589
Mortgage-backed securities:
Government issued or guaranteed		8,755,482		59,497		98,587		8,716,392
Privately issued		28		—		—		28
Other debt securities		136,905		2,402		10,475		128,832
Equity securities		78,161		23,219		424		100,956
		10,938,796		85,154		127,666		10,896,284
Investment securities held to maturity:
Obligations of states and political subdivisions		24,562		109		49		24,622
Mortgage-backed securities:
Government issued or guaranteed		3,187,953		27,236		13,746		3,201,443
Privately issued		135,688		2,574		27,575		110,687
Other debt securities		5,010		—		—		5,010
		3,353,213		29,919		41,370		3,341,762
Other securities		415,028		—		—		415,028
Total	$	14,707,037	$	115,073	$	169,036	$	14,653,074

	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

Investment Ratings of All Privately Issued Mortgage-Backed Securities and Other Debt Securities

As of December 31, 2017, 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	$	27,117	$	27,211	$	15,874	$	—	$	—	$	—	$	11,337
Privately issued mortgage-backed    securities		135,716		110,715		23,459		16		—		87,233		7
Other debt securities		141,915		133,842		6,980		61,054		32,684		—		33,124
Total	$	304,748	$	271,768	$	46,313	$	61,070	$	32,684	$	87,233	$	44,468

Amortized Cost and Estimated Fair Value of Collateralized Mortgage Obligations

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

	December 31
	2017		2016
	(In thousands)
Collateralized mortgage obligations:
Amortized cost	$	138,527	$	162,027
Estimated fair value		113,516		125,848

Amortized Cost and Estimated Fair Value of Debt Securities by Contractual Maturity

At December 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	$	616,908		613,866
Due after one year through five years		1,356,410		1,341,344
Due after five years through ten years		72,642		72,580
Due after ten years		59,165		51,118
		2,105,125		2,078,908
Mortgage-backed securities available for sale		8,755,510		8,716,420
	$	10,860,635		10,795,328
Debt securities held to maturity:
Due in one year or less	$	14,622		14,656
Due after one year through five years		9,844		9,865
Due after five years through ten years		96		101
Due after ten years		5,010		5,010
		29,572		29,632
Mortgage-backed securities held to maturity		3,323,641		3,312,130
	$	3,353,213		3,341,762

Investment Securities in Continuous Unrealized Loss Position

A summary of investment securities that as of December 31, 2017 and 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)
December 31, 2017
Investment securities available for sale:
U.S. Treasury and federal agencies	$	278,132		(1,761	)		1,669,355		(16,417	)
Obligations of states and political subdivisions		—		—			474		(2	)
Mortgage-backed securities:
Government issued or guaranteed		2,106,142		(13,695	)		3,138,841		(84,892	)
Other debt securities		3,067		(26	)		61,159		(10,449	)
Equity securities		—		—			18,162		(424	)
		2,387,341		(15,482	)		4,887,991		(112,184	)
Investment securities held to maturity:
Obligations of states and political subdivisions		2,954		(4	)		6,110		(45	)
Mortgage-backed securities:
Government issued or guaranteed		1,331,759		(7,036	)		265,695		(6,710	)
Privately issued		5,061		(1,216	)		55,255		(26,359	)
		1,339,774		(8,256	)		327,060		(33,114	)
Total	$	3,727,115		(23,738	)		5,215,051		(145,298	)
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	)