Investment Securities Available for Sale and Investment Securities Held to Maturity (Tables)

12 Months Ended

Dec. 31, 2015

Investments, Debt and Equity Securities [Abstract]

Schedule of adjusted cost and approximate fair value of investment securities available for sale and investments held to maturity

The following table presents the adjusted cost and approximate fair value of investment securities available for sale and investment securities held to maturity.

	December 31, 2015
			Gross Unrealized
	Amortized Cost		Gains		Losses		Fair Value
	(In Thousands)
Investment securities available for sale:
Debt securities:
U.S. Treasury and other U.S. government agencies	$	3,232,238	$	4,076	$	24,822	$	3,211,492
Mortgage-backed securities	4,624,441		16,548		50,727		4,590,262
Collateralized mortgage obligations	2,713,075		8,200		16,019		2,705,256
States and political subdivisions	15,492		395		—		15,887
Other	23,914		175		44		24,045
Equity securities	503,540		38		—		503,578
Total	$	11,112,700	$	29,432	$	91,612	$	11,050,520
Investment securities held to maturity:
Collateralized mortgage obligations	$	103,947	$	6,022	$	4,634	$	105,335
Asset-backed securities	24,011		3,002		1,574		25,439
States and political subdivisions	1,128,240		729		82,632		1,046,337
Other	66,478		2,644		2,112		67,010
Total	$	1,322,676	$	12,397	$	90,952	$	1,244,121

	December 31, 2014
			Gross Unrealized
	Amortized Cost		Gains		Losses		Fair Value
	(In Thousands)
Investment securities available for sale:
Debt securities:
U.S. Treasury and other U.S. government agencies	$	2,312,572	$	10,360	$	9,390	$	2,313,542
Mortgage-backed securities	4,399,706		64,371		40,242		4,423,835
Collateralized mortgage obligations	2,475,115		19,385		5,921		2,488,579
States and political subdivisions	460,569		8,008		1,262		467,315
Other	44,225		238		22		44,441
Equity securities	499,522		41		—		499,563
Total	$	10,191,709	$	102,403	$	56,837	$	10,237,275
Investment securities held to maturity:
Collateralized mortgage obligations	$	124,051	$	5,878	$	5,452	$	124,477
Asset-backed securities	39,187		3,568		2,011		40,744
States and political subdivisions	1,112,415		2,143		79,246		1,035,312
Other	72,701		4,920		2,191		75,430
Total	$	1,348,354	$	16,509	$	88,900	$	1,275,963

Schedule of fair value and gross unrealized losses of available for sale and held to maturity securities that were in a loss position

The following table discloses the fair value and the gross unrealized losses of the Company’s available for sale securities and held to maturity securities that were in a loss position at December 31, 2015 and 2014. This information is aggregated by investment category and the length of time the individual securities have been in an unrealized loss position.

	December 31, 2015
	Securities in a loss position for less than 12 months				Securities in a loss position for 12 months or longer				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(In Thousands)
Investment securities available for sale:
Debt securities:
U.S. Treasury and other U.S. government agencies	$	2,081,528	$	16,523	$	460,160	$	8,299	$	2,541,688	$	24,822
Mortgage-backed securities	2,623,761		20,380		1,408,069		30,347		4,031,830		50,727
Collateralized mortgage obligations	1,321,121		10,378		393,210		5,641		1,714,331		16,019
Other	—		—		1,078		44		1,078		44
Total	$	6,026,410	$	47,281	$	2,262,517	$	44,331	$	8,288,927	$	91,612
Investment securities held to maturity:
Collateralized mortgage obligations	$	11,066	$	326	$	52,601	$	4,308	$	63,667	$	4,634
Asset-backed securities	—		—		15,790		1,574		15,790		1,574
States and political subdivisions	73,302		6,533		794,489		76,099		867,791		82,632
Other	—		—		4,015		2,112		4,015		2,112
Total	$	84,368	$	6,859	$	866,895	$	84,093	$	951,263	$	90,952

	December 31, 2014
	Securities in a loss position for less than 12 months				Securities in a loss position for 12 months or longer				Total
	Fair Value		Unrealized Losses		Fair Value		Unrealized Losses		Fair Value		Unrealized Losses
	(In Thousands)
Investment securities available for sale:
Debt securities:
U.S. Treasury and other U.S. government agencies	$	620,794	$	8,220	$	37,220	$	1,170	$	658,014	$	9,390
Mortgage-backed securities	308,734		862		1,915,494		39,380		2,224,228		40,242
Collateralized mortgage obligations	714,173		3,829		146,806		2,092		860,979		5,921
States and political subdivisions	—		—		135,825		1,262		135,825		1,262
Other	—		—		1,099		22		1,099		22
Total	$	1,643,701	$	12,911	$	2,236,444	$	43,926	$	3,880,145	$	56,837
Investment securities held to maturity:
Collateralized mortgage obligations	$	34,400	$	1,099	$	27,389	$	4,353	$	61,789	$	5,452
Asset-backed securities	—		—		23,374		2,011		23,374		2,011
States and political subdivisions	21,688		768		817,570		78,478		839,258		79,246
Other	4,061		2,191		—		—		4,061		2,191
Total	$	60,149	$	4,058	$	868,333	$	84,842	$	928,482	$	88,900

Schedule of activity related to credit losses for debt securities where other than temporary impairments was recognized in other comprehensive income

The following table discloses activity related to credit losses for debt securities where a portion of the OTTI was recognized in other comprehensive income.

	Years Ended December 31,
	2015			2014		2013
	(In Thousands)
Balance, January 1	$	21,123		$	20,943	$	17,318
Reductions for securities paid off during the period (realized)	(331		)	—		(239		)
Additions for the credit component on debt securities in which OTTI was not previously recognized	1,013			—		270
Additions for the credit component on debt securities in which OTTI was previously recognized	647			180		3,594
Balance, December 31	$	22,452		$	21,123	$	20,943

Schedule of investments classified by contractual maturity date

The maturities of the securities portfolios are presented in the following table.

	Amortized Cost		Fair Value
December 31, 2015	(In Thousands)
Investment securities available for sale:
Maturing within one year	$	136,615	$	136,532
Maturing after one but within five years	1,333,506		1,323,471
Maturing after five but within ten years	638,237		637,785
Maturing after ten years	1,163,286		1,153,636
	3,271,644		3,251,424
Mortgage-backed securities and collateralized mortgage obligations	7,337,516		7,295,518
Equity securities	503,540		503,578
Total	$	11,112,700	$	11,050,520
Investment securities held to maturity:
Maturing within one year	$	—	$	—
Maturing after one but within five years	348,793		335,406
Maturing after five but within ten years	245,003		220,384
Maturing after ten years	624,933		582,996
	1,218,729		1,138,786
Collateralized mortgage obligations	103,947		105,335
Total	$	1,322,676	$	1,244,121

Schedule of realized gain (loss) on investments

The gross realized gains and losses recognized on sales of investment securities available for sale are shown in the table below.

	Years Ended December 31,
	2015		2014		2013
	(In Thousands)
Gross gains	$	83,488	$	53,042	$	34,264
Gross losses	1,832		—		2,893
Net realized gains	$	81,656	$	53,042	$	31,371