Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2014
Realized gains and losses
Gross realized gains	$ 2,032	$ 5,368	$ 5,424	$ 7,970
Gross realized losses	(108)	(1)	(124)	(481)
Net realized gains	1,924	5,367	5,300	7,489
Securities available-for-sale:
Amortized Cost	4,943,133		4,943,133		$ 5,894,509
Gross Unrealized Gains	28,511		28,511		32,788
Gross Unrealized Losses	(30,801)		(30,801)		(44,314)
Fair Value	4,940,843		4,940,843		5,882,983
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	3,422,179		3,422,179		3,427,031
Gross Unrealized Gains	58,535		58,535		72,659
Gross Unrealized Losses	(19,772)		(19,772)		(15,043)
Fair Value	3,460,942		3,460,942		3,484,647
Sales of securities available-for-sale	2,600	15,000	400,803	626,055
Sales of securities held-to-maturity			0	0
Interest and dividend income on securities (including trading securities)
Taxable interest income	34,100	37,500	69,100	73,500
Nontaxable interest income	7,100	6,000	14,100	11,500
Dividend income	19	$ 16	30	$ 25
Trading securities	110,136		110,136		173,188
Total securities	8,470,000		8,470,000		9,480,000
Total debt securities
Securities available-for-sale:
Amortized Cost	4,942,469		4,942,469		5,893,001
Gross Unrealized Gains	28,511		28,511		30,800
Gross Unrealized Losses	(30,603)		(30,603)		(43,964)
Fair Value	4,940,377		4,940,377		5,879,837
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	3,422,179		3,422,179
U.S. Treasury
Securities available-for-sale:
Amortized Cost	148,263		148,263		116,919
Gross Unrealized Gains	59		59		17
Gross Unrealized Losses	(14)		(14)		(10)
Fair Value	148,308		148,308		116,926
Federal agency - Debt
Securities available-for-sale:
Amortized Cost	899,379		899,379		1,401,303
Gross Unrealized Gains	762		762		558
Gross Unrealized Losses	(207)		(207)		(3,280)
Fair Value	899,934		899,934		1,398,581
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	323,805		323,805		292,932
Gross Unrealized Gains	6,431		6,431		6,430
Gross Unrealized Losses	(166)		(166)		(255)
Fair Value	330,070		330,070		299,107
Federal agency - MBS
Securities available-for-sale:
Amortized Cost	95,778		95,778		102,939
Gross Unrealized Gains	2,139		2,139		2,601
Gross Unrealized Losses	(1,039)		(1,039)		(1,014)
Fair Value	96,878		96,878		104,526
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	578,603		578,603		553,589
Gross Unrealized Gains	11,447		11,447		13,427
Gross Unrealized Losses	(3,105)		(3,105)		(2,275)
Fair Value	586,945		586,945		564,741
CMOs - Federal agency
Securities available-for-sale:
Amortized Cost	3,295,300		3,295,300		3,599,831
Gross Unrealized Gains	19,497		19,497		19,628
Gross Unrealized Losses	(28,721)		(28,721)		(38,869)
Fair Value	3,286,076		3,286,076		3,580,590
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	1,684,687		1,684,687		1,811,574
Gross Unrealized Gains	26,833		26,833		29,998
Gross Unrealized Losses	(7,480)		(7,480)		(10,292)
Fair Value	1,704,040		1,704,040		1,831,280
CMOs - Non-agency
Securities available-for-sale:
Amortized Cost	22,091		22,091		24,385
Gross Unrealized Gains	21		21		40
Gross Unrealized Losses	(381)		(381)		(411)
Fair Value	21,731		21,731		24,014
State and municipal
Securities available-for-sale:
Amortized Cost	395,350		395,350		473,272
Gross Unrealized Gains	4,946		4,946		6,139
Gross Unrealized Losses	(241)		(241)		(380)
Fair Value	400,055		400,055		479,031
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	749,253		749,253		682,705
Gross Unrealized Gains	13,641		13,641		22,732
Gross Unrealized Losses	(9,021)		(9,021)		(1,997)
Fair Value	753,873		753,873		703,440
Other debt securities
Securities available-for-sale:
Amortized Cost	86,308		86,308		174,352
Gross Unrealized Gains	1,087		1,087		1,817
Fair Value	87,395		87,395		176,169
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	85,831		85,831		86,231
Gross Unrealized Gains	183		183		72
Gross Unrealized Losses					(224)
Fair Value	86,014		86,014		86,079
Equity securities and mutual funds
Securities available-for-sale:
Amortized Cost	664		664		1,508
Gross Unrealized Gains					1,988
Gross Unrealized Losses	(198)		(198)		(350)
Fair Value	$ 466		$ 466		$ 3,146