Investment Securities (Amortized Cost and Fair Values of Investment Securities) (Detail) (USD $)	Sep. 30, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 2,178,698,000	$ 2,387,211,000
Available-for-Sale, Gross Unrealized Gains	39,556,000	28,103,000
Available-for-Sale, Gross Unrealized Losses	25,639,000	40,090,000
Total available-for-sale securities, Fair Value	2,192,615,000	2,375,224,000
U.S. Government Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	93,712,000	110,227,000
Available-for-Sale, Gross Unrealized Gains	135,000	343,000
Available-for-Sale, Gross Unrealized Losses	131,000	422,000
Total available-for-sale securities, Fair Value	93,716,000	110,148,000
Obligations Of States And Political Subdivisions [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	375,529,000	389,199,000
Available-for-Sale, Gross Unrealized Gains	21,471,000	13,386,000
Available-for-Sale, Gross Unrealized Losses	657,000	4,075,000
Total available-for-sale securities, Fair Value	396,343,000	398,510,000
Agency Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	1,630,197,000	1,786,133,000
Available-for-Sale, Gross Unrealized Gains	16,159,000	12,163,000
Available-for-Sale, Gross Unrealized Losses	11,057,000	20,104,000
Total available-for-sale securities, Fair Value	1,635,299,000	1,778,192,000
Non-Agency Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	307,000	572,000
Available-for-Sale, Gross Unrealized Gains	1,000	1,000
Available-for-Sale, Gross Unrealized Losses	4,000	8,000
Total available-for-sale securities, Fair Value	304,000	565,000
Commercial Mortgage Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	5,736,000	8,568,000
Available-for-Sale, Gross Unrealized Gains	104,000	166,000
Available-for-Sale, Gross Unrealized Losses	3,000	0
Total available-for-sale securities, Fair Value	5,837,000	8,734,000
Other Structured Financial Products [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	24,562,000	25,038,000
Available-for-Sale, Gross Unrealized Gains	0	0
Available-for-Sale, Gross Unrealized Losses	12,801,000	13,741,000
Total available-for-sale securities, Fair Value	11,761,000	11,297,000
Other Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	24,078,000	43,156,000
Available-for-Sale, Gross Unrealized Gains	941,000	1,487,000
Available-for-Sale, Gross Unrealized Losses	98,000	557,000
Total available-for-sale securities, Fair Value	24,921,000	44,086,000
Total Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	2,154,121,000	2,362,893,000
Available-for-Sale, Gross Unrealized Gains	38,811,000	27,546,000
Available-for-Sale, Gross Unrealized Losses	24,751,000	38,907,000
Total available-for-sale securities, Fair Value	2,168,181,000	2,351,532,000
Other Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	24,577,000	24,318,000
Available-for-Sale, Gross Unrealized Gains	745,000	557,000
Available-for-Sale, Gross Unrealized Losses	888,000	1,183,000
Total available-for-sale securities, Fair Value	$ 24,434,000	$ 23,692,000