Investment securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Other securities, Amortized cost	$ 300,890	$ 302,648
Other securities, Gross unrealized gains
Other securities, Gross unrealized losses
Other securities, Estimated fair value	300,890	302,648
Investment securities, available for sale, amortized cost	4,288,994	4,643,070
Gross unrealized gains	232,472	253,619
Gross unrealized losses	120,724	157,252
Estimated fair value	4,400,742	4,739,437
Amortized cost for held to maturity	959,199	1,032,276
Gross unrealized gains for held to maturity	33,053	39,534
Gross unrealized losses for held to maturity	81,469	94,927
Estimated fair value for held to maturity	910,783	976,883
Total Amortized cost	5,549,083	5,977,994
Total for Gross unrealized gains	265,525	293,153
Total for Gross unrealized losses	202,193	252,179
Total for Estimated fair value	5,612,415	6,018,968
Collateralized Debt Obligations [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	42,957	43,228
Gross unrealized gains	19,799	19,663
Gross unrealized losses	1,038	1,022
Estimated fair value	61,718	61,869
Obligations of States and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	19,545	20,375
Gross unrealized gains	503	534
Gross unrealized losses	7	8
Estimated fair value	20,041	20,901
Amortized cost for held to maturity	180,458	182,103
Gross unrealized gains for held to maturity	7,003	7,647
Gross unrealized losses for held to maturity	31	27
Estimated fair value for held to maturity	187,430	189,723
Mortgage-backed Securities [Member] \| Government Issued or Guaranteed [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	2,855,716	3,163,210
Gross unrealized gains	174,372	208,060
Gross unrealized losses	178	229
Estimated fair value	3,029,910	3,371,041
Amortized cost for held to maturity	532,074	597,340
Gross unrealized gains for held to maturity	26,003	31,727
Gross unrealized losses for held to maturity
Estimated fair value for held to maturity	558,077	629,067
Mortgage-backed Securities [Member] \| Privately issued [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Amortized cost for held to maturity	236,632	242,378
Gross unrealized gains for held to maturity	47	160
Gross unrealized losses for held to maturity	81,438	94,900
Estimated fair value for held to maturity	155,241	147,638
Mortgage-backed Securities [Member] \| Privately Issued Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	1,077,879	1,133,639
Gross unrealized gains	5,799	4,894
Gross unrealized losses	97,989	125,647
Estimated fair value	985,689	1,012,886
Mortgage-backed Securities [Member] \| Privately Issued Commercial Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	6,414	8,648
Gross unrealized gains	1,144	2,378
Gross unrealized losses		26
Estimated fair value	7,558	11,000
Other Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	136,645	136,603
Gross unrealized gains	2,371	2,247
Gross unrealized losses	21,505	26,900
Estimated fair value	117,511	111,950
Amortized cost for held to maturity	10,035	10,455
Gross unrealized gains for held to maturity
Gross unrealized losses for held to maturity
Estimated fair value for held to maturity	10,035	10,455
U.S. Treasury and Federal Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	38,907	38,422
Gross unrealized gains	793	922
Gross unrealized losses
Estimated fair value	39,700	39,344
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Investment securities, available for sale, amortized cost	110,931	98,945
Gross unrealized gains	27,691	14,921
Gross unrealized losses	7	3,420
Estimated fair value	$ 138,615	$ 110,446

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Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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