Securities (Details) (USD $)	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013	Dec. 31, 2013
Securities
Total securities	$ 9,210,000,000		$ 9,210,000,000		$ 9,280,000,000
Trading securities, at fair value	125,910,000		125,910,000		82,357,000
Securities available-for-sale:
Amortized Cost	5,641,894,000		5,641,894,000		6,267,691,000
Gross Unrealized Gains	41,111,000		41,111,000		56,067,000
Gross Unrealized Losses	(53,829,000)		(53,829,000)		(82,641,000)
Fair Value	5,629,176,000		5,629,176,000		6,241,117,000
Proceeds from sales of securities	1,000,000	584,700,000	627,102,000	1,835,775,000
Securities held-to-maturity:
Total of debt securities held-to-maturity, amortized cost	3,450,551,000		3,450,551,000		2,957,843,000
Gross Unrealized Gains	43,910,000		43,910,000		3,398,000
Gross Unrealized Losses	(32,473,000)		(32,473,000)		(77,306,000)
Fair Value	3,461,988,000		3,461,988,000		2,883,935,000
Realized gains and losses
Gross realized gains	19,000	17,533,000	7,989,000	24,369,000
Gross realized losses	(5,000)	(11,745,000)	(486,000)	(11,745,000)
Net realized gains	14,000	5,788,000	7,503,000	12,624,000
Interest and dividend income on securities (including trading securities)
Taxable interest income	37,500,000	35,500,000	111,000,000	112,100,000
Nontaxable interest income	6,400,000	4,500,000	17,900,000	13,300,000
Dividend income	22,000	19,000	47,000	100,000
Total debt securities
Securities available-for-sale:
Amortized Cost	5,641,273,000		5,641,273,000		6,267,354,000
Gross Unrealized Gains	36,420,000		36,420,000		47,961,000
Gross Unrealized Losses	(53,829,000)		(53,829,000)		(82,641,000)
Fair Value	5,623,864,000		5,623,864,000		6,232,674,000
U.S. Treasury
Securities available-for-sale:
Amortized Cost	46,716,000		46,716,000		35,312,000
Gross Unrealized Gains	49,000		49,000		23,000
Fair Value	46,765,000		46,765,000		35,335,000
Federal agency - Debt
Securities available-for-sale:
Amortized Cost	1,329,891,000		1,329,891,000		1,417,509,000
Gross Unrealized Gains	581,000		581,000		938,000
Gross Unrealized Losses	(3,744,000)		(3,744,000)		(7,911,000)
Fair Value	1,326,728,000		1,326,728,000		1,410,536,000
Securities held-to-maturity:
Total of debt securities held-to-maturity, amortized cost	296,900,000		296,900,000		178,413,000
Gross Unrealized Gains	4,013,000		4,013,000		133,000
Gross Unrealized Losses	(1,154,000)		(1,154,000)		(5,122,000)
Fair Value	299,759,000		299,759,000		173,424,000
Federal agency - MBS
Securities available-for-sale:
Amortized Cost	105,992,000		105,992,000		156,399,000
Gross Unrealized Gains	2,826,000		2,826,000		3,615,000
Gross Unrealized Losses	(1,842,000)		(1,842,000)		(2,788,000)
Fair Value	106,976,000		106,976,000		157,226,000
Securities held-to-maturity:
Total of debt securities held-to-maturity, amortized cost	582,365,000		582,365,000		445,360,000
Gross Unrealized Gains	7,137,000		7,137,000		1,005,000
Gross Unrealized Losses	(5,186,000)		(5,186,000)		(11,930,000)
Fair Value	584,316,000		584,316,000		434,435,000
CMOs - Federal agency
Securities available-for-sale:
Amortized Cost	3,583,667,000		3,583,667,000		4,037,348,000
Gross Unrealized Gains	23,201,000		23,201,000		30,721,000
Gross Unrealized Losses	(47,790,000)		(47,790,000)		(70,771,000)
Fair Value	3,559,078,000		3,559,078,000		3,997,298,000
Securities held-to-maturity:
Total of debt securities held-to-maturity, amortized cost	1,847,655,000		1,847,655,000		1,781,219,000
Gross Unrealized Gains	14,816,000		14,816,000		1,839,000
Gross Unrealized Losses	(23,018,000)		(23,018,000)		(40,621,000)
Fair Value	1,839,453,000		1,839,453,000		1,742,437,000
CMOs - Non-agency
Securities available-for-sale:
Amortized Cost	25,521,000		25,521,000		38,383,000
Gross Unrealized Gains	128,000		128,000		127,000
Gross Unrealized Losses	(367,000)		(367,000)		(1,048,000)
Fair Value	25,282,000		25,282,000		37,462,000
State and municipal
Securities available-for-sale:
Amortized Cost	374,948,000		374,948,000		407,312,000
Gross Unrealized Gains	7,131,000		7,131,000		8,806,000
Gross Unrealized Losses	(86,000)		(86,000)		(123,000)
Fair Value	381,993,000		381,993,000		415,995,000
Securities held-to-maturity:
Total of debt securities held-to-maturity, amortized cost	625,860,000		625,860,000		454,155,000
Gross Unrealized Gains	17,884,000		17,884,000		421,000
Gross Unrealized Losses	(2,925,000)		(2,925,000)		(19,014,000)
Fair Value	640,819,000		640,819,000		435,562,000
Other debt securities
Securities available-for-sale:
Amortized Cost	174,538,000		174,538,000		175,091,000
Gross Unrealized Gains	2,504,000		2,504,000		3,731,000
Fair Value	177,042,000		177,042,000		178,822,000
Securities held-to-maturity:
Total of debt securities held-to-maturity, amortized cost	97,771,000		97,771,000		98,696,000
Gross Unrealized Gains	60,000		60,000
Gross Unrealized Losses	(190,000)		(190,000)		(619,000)
Fair Value	97,641,000		97,641,000		98,077,000
Equity securities and mutual funds
Securities available-for-sale:
Amortized Cost	621,000		621,000		337,000
Gross Unrealized Gains	4,691,000		4,691,000		8,106,000
Fair Value	$ 5,312,000		$ 5,312,000		$ 8,443,000