Investment Securities - Amortized Cost and Estimated Fair Value of Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Investment Securities [Line Items]
Amortized Cost	$ 2,173,946	$ 2,130,029
Gross Unrealized Holding Gain	85,074	72,703
Gross Unrealized Holding Loss	(1,513)	(1,206)
Fair Value	2,257,507	2,201,526
Total Percent	100.00%	100.00%
US Government-sponsored Enterprises Debt Securities [Member]
Investment Securities [Line Items]
Amortized Cost	93,660	46,273
Gross Unrealized Holding Gain	247	234
Gross Unrealized Holding Loss	(64)
Fair Value	93,843	46,507
Total Percent	4.16%	2.11%
Residential mortgage-backed securities [Member]
Investment Securities [Line Items]
Amortized Cost	839,474	869,847
Gross Unrealized Holding Gain	30,123	18,487
Gross Unrealized Holding Loss		(334)
Fair Value	869,597	888,000
Total Percent	38.52%	40.33%
CMO's / REMIC's - Residential [Member]
Investment Securities [Line Items]
Amortized Cost	643,150	594,866
Gross Unrealized Holding Gain	7,944	10,307
Gross Unrealized Holding Loss	(1,325)	(665)
Fair Value	649,769	604,508
Total Percent	28.78%	27.46%
Municipal Bonds [Member]
Investment Securities [Line Items]
Amortized Cost	592,662	608,575
Gross Unrealized Holding Gain	46,685	43,665
Gross Unrealized Holding Loss	(124)	(203)
Fair Value	639,223	652,037
Total Percent	28.32%	29.62%
Other Securities [Member]
Investment Securities [Line Items]
Amortized Cost	5,000	10,468
Gross Unrealized Holding Gain	75	10
Gross Unrealized Holding Loss		(4)
Fair Value	$ 5,075	$ 10,474
Total Percent	0.22%	0.48%