Securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011	Dec. 31, 2011
Securities
Total securities	$ 7,920,000,000		$ 8,100,000,000
Trading securities, at fair value	82,589,000		61,975,000
Securities available-for-sale:
Amortized Cost	6,699,181,000		7,445,999,000
Gross Unrealized Gains	155,837,000		149,080,000
Gross Unrealized Losses	(16,308,000)		(23,178,000)
Fair Value	6,838,710,000		7,571,901,000
Securities held-to-maturity:
Amortized Cost	996,613,000		467,680,000
Gross Unrealized Gains	6,115,000		6,455,000
Gross Unrealized Losses	(6,273,000)		(232,000)
Fair Value	996,455,000		473,903,000
Proceeds from sales of securities	5,173,000	6,094,000
Realized gains and losses
Gross realized gains	496,000	160,000
Gross realized losses	(47,000)	(30,000)
Net realized gains	449,000	130,000
Interest and dividend income on securities (including trading securities)
Taxable interest income	41,500,000	34,300,000
Nontaxable interest income	3,800,000	2,900,000
Dividend income	100,000	200,000
Total debt securities
Securities available-for-sale:
Amortized Cost	6,698,829,000		7,445,647,000
Gross Unrealized Gains	155,039,000		148,205,000
Gross Unrealized Losses	(16,308,000)		(23,178,000)
Fair Value	6,837,560,000		7,570,674,000
U.S. Treasury
Securities available-for-sale:
Amortized Cost	19,217,000		19,163,000
Gross Unrealized Gains	8,000		24,000
Gross Unrealized Losses	(23,000)		(5,000)
Fair Value	19,202,000		19,182,000
Federal agency - Debt
Securities available-for-sale:
Amortized Cost	1,046,035,000		1,967,928,000
Gross Unrealized Gains	5,348,000		6,230,000
Gross Unrealized Losses	(35,000)		(296,000)
Fair Value	1,051,348,000		1,973,862,000
Securities held-to-maturity:
Amortized Cost	104,268,000		40,423,000
Gross Unrealized Gains	394,000		780,000
Gross Unrealized Losses	(66,000)
Fair Value	104,596,000		41,203,000
Federal agency - MBS
Securities available-for-sale:
Amortized Cost	690,183,000		650,091,000
Gross Unrealized Gains	30,580,000		31,040,000
Gross Unrealized Losses	(173,000)		(87,000)
Fair Value	720,590,000		681,044,000
Securities held-to-maturity:
Amortized Cost	173,380,000		75,231,000
Gross Unrealized Gains	1,794,000		1,632,000
Gross Unrealized Losses	(427,000)
Fair Value	174,747,000		76,863,000
CMOs - Federal agency
Securities available-for-sale:
Amortized Cost	4,304,148,000		4,239,205,000
Gross Unrealized Gains	97,498,000		89,926,000
Gross Unrealized Losses	(1,328,000)		(2,224,000)
Fair Value	4,400,318,000		4,326,907,000
Securities held-to-maturity:
Amortized Cost	590,931,000		292,547,000
Gross Unrealized Gains	2,496,000		2,580,000
Gross Unrealized Losses	(3,918,000)		(195,000)
Fair Value	589,509,000		294,932,000
CMOs - Non-agency
Securities available-for-sale:
Amortized Cost	74,737,000		79,999,000
Gross Unrealized Gains	809,000		322,000
Gross Unrealized Losses	(6,959,000)		(11,320,000)
Fair Value	68,587,000		69,001,000
State and municipal
Securities available-for-sale:
Amortized Cost	378,541,000		383,210,000
Gross Unrealized Gains	17,379,000		18,767,000
Gross Unrealized Losses	(197,000)		(373,000)
Fair Value	395,723,000		401,604,000
Securities held-to-maturity:
Amortized Cost	128,034,000		59,479,000
Gross Unrealized Gains	1,431,000		1,463,000
Gross Unrealized Losses	(1,862,000)		(37,000)
Fair Value	127,603,000		60,905,000
Other debt securities
Securities available-for-sale:
Amortized Cost	185,968,000		106,051,000
Gross Unrealized Gains	3,417,000		1,896,000
Gross Unrealized Losses	(7,593,000)		(8,873,000)
Fair Value	181,792,000		99,074,000
Equity securities and mutual funds
Securities available-for-sale:
Amortized Cost	352,000		352,000
Gross Unrealized Gains	798,000		875,000
Fair Value	$ 1,150,000		$ 1,227,000