Investment Securities (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010
Available-for-sale securities
Amortized Cost	$ 6,250,759,000	$ 4,668,089,000	$ 6,250,759,000	$ 4,668,089,000	$ 5,658,120,000
Gross Unrealized Gains	118,432,000	121,256,000	118,432,000	121,256,000	100,404,000
Gross Unrealized Losses	(21,136,000)	(28,202,000)	(21,136,000)	(28,202,000)	(37,849,000)
Fair Value	6,348,055,000	4,761,143,000	6,348,055,000	4,761,143,000	5,720,675,000
Proceeds from sales of securities	47,200,000	24,400,000	53,304,000	432,021,000
Realized gains and losses
Gross realized gains	2,621,000	491,000	2,781,000	4,993,000
Gross realized losses	(932,000)	(136,000)	(962,000)	(2,504,000)
Net realized gains	1,689,000	355,000	1,819,000	2,489,000
Interest and dividend income on securities available-for-sale
Taxable interest income	36,300,000	29,600,000	70,600,000	58,400,000
Nontaxable interest income	2,900,000	3,100,000	5,800,000	6,200,000
Dividend income	200,000	200,000	300,000	500,000
Total debt securities
Available-for-sale securities
Amortized Cost	6,248,671,000	4,659,961,000	6,248,671,000	4,659,961,000	5,651,575,000
Gross Unrealized Gains	114,408,000	117,829,000	114,408,000	117,829,000	96,513,000
Gross Unrealized Losses	(21,136,000)	(28,202,000)	(21,136,000)	(28,202,000)	(37,849,000)
Fair Value	6,341,943,000	4,749,588,000	6,341,943,000	4,749,588,000	5,710,239,000
U.S. Treasury
Available-for-sale securities
Amortized Cost	13,036,000	19,096,000	13,036,000	19,096,000	14,070,000
Gross Unrealized Gains	40,000	50,000	40,000	50,000	47,000
Gross Unrealized Losses		(1,000)		(1,000)	(4,000)
Fair Value	13,076,000	19,145,000	13,076,000	19,145,000	14,113,000
Federal agency - Debt
Available-for-sale securities
Amortized Cost	1,841,579,000	1,084,703,000	1,841,579,000	1,084,703,000	1,142,520,000
Gross Unrealized Gains	6,534,000	6,432,000	6,534,000	6,432,000	5,029,000
Gross Unrealized Losses	(881,000)	(289,000)	(881,000)	(289,000)	(5,221,000)
Fair Value	1,847,232,000	1,090,846,000	1,847,232,000	1,090,846,000	1,142,328,000
Federal agency - MBS
Available-for-sale securities
Amortized Cost	518,421,000	447,363,000	518,421,000	447,363,000	540,768,000
Gross Unrealized Gains	17,961,000	19,350,000	17,961,000	19,350,000	13,379,000
Gross Unrealized Losses	(1,656,000)		(1,656,000)		(2,801,000)
Fair Value	534,726,000	466,713,000	534,726,000	466,713,000	551,346,000
CMOs - Federal agency
Available-for-sale securities
Amortized Cost	3,383,652,000	2,455,952,000	3,383,652,000	2,455,952,000	3,442,238,000
Gross Unrealized Gains	73,040,000	74,401,000	73,040,000	74,401,000	65,494,000
Gross Unrealized Losses	(2,791,000)	(2,116,000)	(2,791,000)	(2,116,000)	(10,585,000)
Fair Value	3,453,901,000	2,528,237,000	3,453,901,000	2,528,237,000	3,497,147,000
CMOs - Non-agency
Available-for-sale securities
Amortized Cost	98,596,000	234,330,000	98,596,000	234,330,000	126,819,000
Gross Unrealized Gains	591,000	1,753,000	591,000	1,753,000	1,147,000
Gross Unrealized Losses	(8,104,000)	(19,005,000)	(8,104,000)	(19,005,000)	(9,671,000)
Fair Value	91,083,000	217,078,000	91,083,000	217,078,000	118,295,000
State and municipal
Available-for-sale securities
Amortized Cost	344,561,000	347,469,000	344,561,000	347,469,000	334,596,000
Gross Unrealized Gains	13,496,000	13,120,000	13,496,000	13,120,000	9,399,000
Gross Unrealized Losses	(253,000)	(167,000)	(253,000)	(167,000)	(615,000)
Fair Value	357,804,000	360,422,000	357,804,000	360,422,000	343,380,000
Other debt securities
Available-for-sale securities
Amortized Cost	48,826,000	71,048,000	48,826,000	71,048,000	50,564,000
Gross Unrealized Gains	2,746,000	2,723,000	2,746,000	2,723,000	2,018,000
Gross Unrealized Losses	(7,451,000)	(6,624,000)	(7,451,000)	(6,624,000)	(8,952,000)
Fair Value	44,121,000	67,147,000	44,121,000	67,147,000	43,630,000
Equity securities and mutual funds
Available-for-sale securities
Amortized Cost	2,088,000	8,128,000	2,088,000	8,128,000	6,545,000
Gross Unrealized Gains	4,024,000	3,427,000	4,024,000	3,427,000	3,891,000
Fair Value	$ 6,112,000	$ 11,555,000	$ 6,112,000	$ 11,555,000	$ 10,436,000