Securities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011	Dec. 31, 2011
Securities
Total securities	$ 9,110,000,000		$ 9,110,000,000		$ 8,100,000,000
Trading securities, at fair value	64,749,000		64,749,000		61,975,000
Securities available-for-sale:
Amortized Cost	7,710,762,000		7,710,762,000		7,445,999,000
Gross Unrealized Gains	174,868,000		174,868,000		149,080,000
Gross Unrealized Losses	(13,566,000)		(13,566,000)		(23,178,000)
Fair Value	7,872,064,000		7,872,064,000		7,571,901,000
Proceeds from sales of securities	1,000,000	48,200,000	6,216,000	101,548,000
Securities held-to-maturity:
Amortized Cost	1,174,161,000		1,174,161,000		467,680,000
Gross Unrealized Gains	48,712,000		48,712,000		6,455,000
Gross Unrealized Losses	(152,000)		(152,000)		(232,000)
Fair Value	1,222,721,000		1,222,721,000		473,903,000
Realized gains and losses
Gross realized gains	1,315,000	3,897,000	1,851,000	6,678,000
Gross realized losses	(459,000)	(377,000)	(825,000)	(1,339,000)
Net realized gains	856,000	3,520,000	1,026,000	5,339,000
Interest and dividend income on securities (including trading securities)
Taxable interest income	40,000,000	37,700,000	120,800,000	108,400,000
Nontaxable interest income	4,100,000	3,000,000	12,000,000	8,900,000
Dividend income	100,000	200,000	300,000	700,000
Total debt securities
Securities available-for-sale:
Amortized Cost	7,710,426,000		7,710,426,000		7,445,647,000
Gross Unrealized Gains	174,383,000		174,383,000		148,205,000
Gross Unrealized Losses	(13,566,000)		(13,566,000)		(23,178,000)
Fair Value	7,871,243,000		7,871,243,000		7,570,674,000
U.S. Treasury
Securities available-for-sale:
Amortized Cost	20,352,000		20,352,000		19,163,000
Gross Unrealized Gains	6,000		6,000		24,000
Gross Unrealized Losses	(3,000)		(3,000)		(5,000)
Fair Value	20,355,000		20,355,000		19,182,000
Federal agency - Debt
Securities available-for-sale:
Amortized Cost	1,542,162,000		1,542,162,000		1,967,928,000
Gross Unrealized Gains	5,169,000		5,169,000		6,230,000
Gross Unrealized Losses	(31,000)		(31,000)		(296,000)
Fair Value	1,547,300,000		1,547,300,000		1,973,862,000
Securities held-to-maturity:
Amortized Cost	96,389,000		96,389,000		40,423,000
Gross Unrealized Gains	2,960,000		2,960,000		780,000
Fair Value	99,349,000		99,349,000		41,203,000
Federal agency - MBS
Securities available-for-sale:
Amortized Cost	612,713,000		612,713,000		650,091,000
Gross Unrealized Gains	45,222,000		45,222,000		31,040,000
Gross Unrealized Losses					(87,000)
Fair Value	657,935,000		657,935,000		681,044,000
Securities held-to-maturity:
Amortized Cost	218,085,000		218,085,000		75,231,000
Gross Unrealized Gains	11,558,000		11,558,000		1,632,000
Fair Value	229,643,000		229,643,000		76,863,000
CMOs - Federal agency
Securities available-for-sale:
Amortized Cost	4,755,996,000		4,755,996,000		4,239,205,000
Gross Unrealized Gains	95,410,000		95,410,000		89,926,000
Gross Unrealized Losses	(3,935,000)		(3,935,000)		(2,224,000)
Fair Value	4,847,471,000		4,847,471,000		4,326,907,000
Securities held-to-maturity:
Amortized Cost	660,196,000		660,196,000		292,547,000
Gross Unrealized Gains	28,830,000		28,830,000		2,580,000
Gross Unrealized Losses					(195,000)
Fair Value	689,026,000		689,026,000		294,932,000
CMOs - Non-agency
Securities available-for-sale:
Amortized Cost	66,431,000		66,431,000		79,999,000
Gross Unrealized Gains	1,070,000		1,070,000		322,000
Gross Unrealized Losses	(3,012,000)		(3,012,000)		(11,320,000)
Fair Value	64,489,000		64,489,000		69,001,000
State and municipal
Securities available-for-sale:
Amortized Cost	406,127,000		406,127,000		383,210,000
Gross Unrealized Gains	19,177,000		19,177,000		18,767,000
Gross Unrealized Losses	(135,000)		(135,000)		(373,000)
Fair Value	425,169,000		425,169,000		401,604,000
Securities held-to-maturity:
Amortized Cost	199,491,000		199,491,000		59,479,000
Gross Unrealized Gains	5,364,000		5,364,000		1,463,000
Gross Unrealized Losses	(152,000)		(152,000)		(37,000)
Fair Value	204,703,000		204,703,000		60,905,000
Other debt securities
Securities available-for-sale:
Amortized Cost	306,645,000		306,645,000		106,051,000
Gross Unrealized Gains	8,329,000		8,329,000		1,896,000
Gross Unrealized Losses	(6,450,000)		(6,450,000)		(8,873,000)
Fair Value	308,524,000		308,524,000		99,074,000
Equity securities and mutual funds
Securities available-for-sale:
Amortized Cost	336,000		336,000		352,000
Gross Unrealized Gains	485,000		485,000		875,000
Fair Value	$ 821,000		$ 821,000		$ 1,227,000