Investment securities (Details) (USD $) In Thousands	Sep. 30, 2011	Dec. 31, 2010
Investment securities available for sale:
Investment securities, available for sale, amortized cost	$ 5,717,180	$ 5,494,377
Gross Unrealized Gains	200,911	230,852
Gross Unrealized Losses	282,213	311,737
Estimated fair value	5,635,878	5,413,492
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	1,161,837	1,324,339
Gross Unrealized Gains for Held-to-maturity	35,449	16,005
Gross Unrealized Losses for Held-to-maturity	100,071	115,091
Estimated fair value for Held-to-maturity	1,097,215	1,225,253
Amortized cost and estimated fair value of investment securities
Other securities, Amortized Cost	376,082	412,709
Other securities, Estimated fair value	376,082	412,709
Total Amortized Cost	7,255,099	7,231,425
Total for Gross Unrealized Gains	236,360	246,857
Total for Gross Unrealized losses	382,284	426,828
Total for Estimated fair value	7,109,175	7,051,454
U.S. Treasury and federal agencies [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	69,668	61,772
Gross Unrealized Gains	1,360	1,680
Gross Unrealized Losses	0	18
Estimated fair value	71,028	63,434
Obligations of states and political subdivisions [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	184,472	191,119
Gross Unrealized Gains for Held-to-maturity	6,942	1,944
Gross Unrealized Losses for Held-to-maturity	19	694
Estimated fair value for Held-to-maturity	191,395	192,369
Obligations of states and political subdivisions [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	45,805	59,921
Gross Unrealized Gains	1,029	561
Gross Unrealized Losses	42	57
Estimated fair value	46,792	60,425
Government issued or guaranteed [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	686,908	808,108
Gross Unrealized Gains for Held-to-maturity	28,507	14,061
Gross Unrealized Losses for Held-to-maturity	0	0
Estimated fair value for Held-to-maturity	715,415	822,169
Government issued or guaranteed [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	3,708,714	3,146,054
Gross Unrealized Gains	168,551	161,298
Gross Unrealized Losses	227	1,111
Estimated fair value	3,877,038	3,306,241
Privately issued [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	278,433	312,537
Gross Unrealized Gains for Held-to-maturity	0
Gross Unrealized Losses for Held-to-maturity	100,052	114,397
Estimated fair value for Held-to-maturity	178,381	198,140
Privately issued residential [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	1,441,491	1,677,064
Gross Unrealized Gains	7,136	10,578
Gross Unrealized Losses	233,549	252,081
Estimated fair value	1,215,078	1,435,561
Privately issued commercial [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	19,446	25,357
Gross Unrealized Gains	0
Gross Unrealized Losses	2,617	2,950
Estimated fair value	16,829	22,407
Collateralized Debt Obligations [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	44,064	95,080
Gross Unrealized Gains	10,141	24,754
Gross Unrealized Losses	2,851	9,078
Estimated fair value	51,354	110,756
Other Debt Securities [Member]
Investment securities held to maturity:
Amortized Cost for Held-to-maturity	12,024	12,575
Gross Unrealized Gains for Held-to-maturity	0
Gross Unrealized Losses for Held-to-maturity	0
Estimated fair value for Held-to-maturity	12,024	12,575
Other Debt Securities [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	216,635	310,017
Gross Unrealized Gains	4,164	26,883
Gross Unrealized Losses	39,946	38,000
Estimated fair value	180,853	298,900
Equity Securities [Member]
Investment securities available for sale:
Investment securities, available for sale, amortized cost	171,357	119,112
Gross Unrealized Gains	8,530	5,098
Gross Unrealized Losses	2,981	8,442
Estimated fair value	$ 176,906	$ 115,768

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