Securities (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Securities
Total securities	$ 9,190,000,000		$ 10,720,000,000
Trading securities, at fair value	53,526,000		115,059,000
Securities available-for-sale:
Amortized Cost	7,610,489,000		9,057,238,000
Gross Unrealized Gains	147,053,000		163,295,000
Gross Unrealized Losses	(19,491,000)		(14,544,000)
Fair Value	7,738,051,000		9,205,989,000
Proceeds from sales of securities	1,034,250,000	5,173,000
Securities held-to-maturity:
Amortized Cost	1,400,890,000		1,398,403,000
Gross Unrealized Gains	39,423,000		49,617,000
Gross Unrealized Losses	(5,477,000)		(1,421,000)
Fair Value	1,434,836,000		1,446,599,000
Proceeds from held-to-maturity	0	0
Realized gains and losses
Gross realized gains	1,046,000	496,000
Gross realized losses		(47,000)
Net realized gains	1,046,000	449,000
Interest and dividend income on securities (including trading securities)
Taxable interest income	39,800,000	41,500,000
Nontaxable interest income	4,400,000	3,800,000
Dividend income	19,000	41,000
Total debt securities
Securities available-for-sale:
Amortized Cost	7,610,152,000		9,056,902,000
Gross Unrealized Gains	144,361,000		161,930,000
Gross Unrealized Losses	(19,491,000)		(14,544,000)
Fair Value	7,735,022,000		9,204,288,000
U.S. Treasury
Securities available-for-sale:
Amortized Cost	30,353,000		20,393,000
Gross Unrealized Gains	17,000		7,000
Gross Unrealized Losses			(3,000)
Fair Value	30,370,000		20,397,000
Federal agency - Debt
Securities available-for-sale:
Amortized Cost	1,111,725,000		2,344,374,000
Gross Unrealized Gains	2,951,000		5,031,000
Gross Unrealized Losses	(73,000)		(203,000)
Fair Value	1,114,603,000		2,349,202,000
Securities held-to-maturity:
Amortized Cost	95,935,000		97,183,000
Gross Unrealized Gains	3,596,000		4,032,000
Fair Value	99,531,000		101,215,000
Federal agency - MBS
Securities available-for-sale:
Amortized Cost	611,286,000		653,428,000
Gross Unrealized Gains	35,131,000		39,673,000
Gross Unrealized Losses	(937,000)		(69,000)
Fair Value	645,480,000		693,032,000
Securities held-to-maturity:
Amortized Cost	305,881,000		303,642,000
Gross Unrealized Gains	8,867,000		11,490,000
Gross Unrealized Losses	(946,000)		(182,000)
Fair Value	313,802,000		314,950,000
CMOs - Federal agency
Securities available-for-sale:
Amortized Cost	5,068,740,000		5,233,126,000
Gross Unrealized Gains	81,888,000		91,165,000
Gross Unrealized Losses	(11,397,000)		(6,038,000)
Fair Value	5,139,231,000		5,318,253,000
Securities held-to-maturity:
Amortized Cost	741,007,000		745,980,000
Gross Unrealized Gains	23,680,000		28,973,000
Gross Unrealized Losses	(1,960,000)		(382,000)
Fair Value	762,727,000		774,571,000
CMOs - Non-agency
Securities available-for-sale:
Amortized Cost	51,201,000		62,975,000
Gross Unrealized Gains	464,000		662,000
Gross Unrealized Losses	(889,000)		(2,124,000)
Fair Value	50,776,000		61,513,000
State and municipal
Securities available-for-sale:
Amortized Cost	430,002,000		437,266,000
Gross Unrealized Gains	16,188,000		17,447,000
Gross Unrealized Losses	(1,020,000)		(239,000)
Fair Value	445,170,000		454,474,000
Securities held-to-maturity:
Amortized Cost	258,067,000		251,598,000
Gross Unrealized Gains	3,280,000		5,122,000
Gross Unrealized Losses	(2,571,000)		(857,000)
Fair Value	258,776,000		255,863,000
Other debt securities
Securities available-for-sale:
Amortized Cost	306,845,000		305,340,000
Gross Unrealized Gains	7,722,000		7,945,000
Gross Unrealized Losses	(5,175,000)		(5,868,000)
Fair Value	309,392,000		307,417,000
Equity securities and mutual funds
Securities available-for-sale:
Amortized Cost	337,000		336,000
Gross Unrealized Gains	2,692,000		1,365,000
Fair Value	$ 3,029,000		$ 1,701,000