Securities (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014	Dec. 31, 2014
Realized gains and losses
Gross realized gains	$ 50	$ 19	$ 5,475	$ 7,989
Gross realized losses	(20)	(5)	(144)	(486)
Net realized gains	30	14	5,331	7,503
Securities available-for-sale:
Amortized Cost	5,304,000		5,304,000		$ 5,894,509
Gross Unrealized Gains	28,101		28,101		32,788
Gross Unrealized Losses	(22,914)		(22,914)		(44,314)
Fair Value	5,309,187		5,309,187		5,882,983
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	3,506,455		3,506,455		3,427,031
Gross Unrealized Gains	97,547		97,547		72,659
Gross Unrealized Losses	(5,100)		(5,100)		(15,043)
Fair Value	3,598,902		3,598,902		3,484,647
Sales of securities available-for-sale	700	1,000	401,534	627,102
Sales of securities held-to-maturity			0	0
Interest and dividend income on securities (including trading securities)
Taxable interest income	33,400	37,500	102,500	111,000
Nontaxable interest income	7,200	6,400	21,300	17,900
Dividend income	22	$ 22	52	$ 47
Trading securities	154,706		154,706		173,188
Total securities	8,970,000		8,970,000		9,480,000
Total debt securities
Securities available-for-sale:
Amortized Cost					5,893,001
Gross Unrealized Gains					30,800
Gross Unrealized Losses					(43,964)
Fair Value	5,309,187		5,309,187		5,879,837
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	3,506,455		3,506,455
U.S. Treasury
Securities available-for-sale:
Amortized Cost	390,098		390,098		116,919
Gross Unrealized Gains	232		232		17
Gross Unrealized Losses	(1)		(1)		(10)
Fair Value	390,329		390,329		116,926
Federal agency - Debt
Securities available-for-sale:
Amortized Cost	285,616		285,616		1,401,303
Gross Unrealized Gains	758		758		558
Gross Unrealized Losses	(14)		(14)		(3,280)
Fair Value	286,360		286,360		1,398,581
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	445,721		445,721		292,932
Gross Unrealized Gains	11,742		11,742		6,430
Gross Unrealized Losses	(27)		(27)		(255)
Fair Value	457,436		457,436		299,107
Federal agency - MBS
Securities available-for-sale:
Amortized Cost	91,690		91,690		102,939
Gross Unrealized Gains	1,851		1,851		2,601
Gross Unrealized Losses	(660)		(660)		(1,014)
Fair Value	92,881		92,881		104,526
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	575,071		575,071		553,589
Gross Unrealized Gains	19,039		19,039		13,427
Gross Unrealized Losses	(227)		(227)		(2,275)
Fair Value	593,883		593,883		564,741
CMOs - Federal agency
Securities available-for-sale:
Amortized Cost	3,538,000		3,538,000		3,599,831
Gross Unrealized Gains	19,867		19,867		19,628
Gross Unrealized Losses	(21,739)		(21,739)		(38,869)
Fair Value	3,536,128		3,536,128		3,580,590
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	1,630,388		1,630,388		1,811,574
Gross Unrealized Gains	44,862		44,862		29,998
Gross Unrealized Losses	(2,057)		(2,057)		(10,292)
Fair Value	1,673,193		1,673,193		1,831,280
CMOs - Non-agency
Securities available-for-sale:
Amortized Cost	20,749		20,749		24,385
Gross Unrealized Gains	25		25		40
Gross Unrealized Losses	(343)		(343)		(411)
Fair Value	20,431		20,431		24,014
State and municipal
Securities available-for-sale:
Amortized Cost	356,623		356,623		473,272
Gross Unrealized Gains	4,249		4,249		6,139
Gross Unrealized Losses	(128)		(128)		(380)
Fair Value	360,744		360,744		479,031
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	769,646		769,646		682,705
Gross Unrealized Gains	21,704		21,704		22,732
Gross Unrealized Losses	(2,788)		(2,788)		(1,997)
Fair Value	788,562		788,562		703,440
Other debt securities
Securities available-for-sale:
Amortized Cost	621,224		621,224		174,352
Gross Unrealized Gains	1,119		1,119		1,817
Gross Unrealized Losses	(29)		(29)
Fair Value	622,314		622,314		176,169
Securities held-to-maturity:
Held-to-maturity Securities, Amortized Cost	85,629		85,629		86,231
Gross Unrealized Gains	200		200		72
Gross Unrealized Losses	(1)		(1)		(224)
Fair Value	$ 85,828		$ 85,828		86,079
Equity securities and mutual funds
Securities available-for-sale:
Amortized Cost					1,508
Gross Unrealized Gains					1,988
Gross Unrealized Losses					(350)
Fair Value					$ 3,146