Investment Securities (Amortized Cost and Fair Values of Investment Securities) (Detail) (USD $)	Mar. 31, 2014	Dec. 31, 2013
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	$ 2,295,100,000	$ 2,387,211,000
Available-for-Sale, Gross Unrealized Gains	35,932,000	28,103,000
Available-for-Sale, Gross Unrealized Losses	31,797,000	40,090,000
Total available-for-sale securities, Fair Value	2,299,235,000	2,375,224,000
U.S. Government Agencies [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	110,176,000	110,227,000
Available-for-Sale, Gross Unrealized Gains	388,000	343,000
Available-for-Sale, Gross Unrealized Losses	332,000	422,000
Total available-for-sale securities, Fair Value	110,232,000	110,148,000
Obligations Of States And Political Subdivisions [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	385,404,000	389,199,000
Available-for-Sale, Gross Unrealized Gains	17,158,000	13,386,000
Available-for-Sale, Gross Unrealized Losses	2,536,000	4,075,000
Total available-for-sale securities, Fair Value	400,026,000	398,510,000
Agency Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	1,700,182,000	1,786,133,000
Available-for-Sale, Gross Unrealized Gains	15,086,000	12,163,000
Available-for-Sale, Gross Unrealized Losses	13,671,000	20,104,000
Total available-for-sale securities, Fair Value	1,701,597,000	1,778,192,000
Non-Agency Residential Mortgage-Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	567,000	572,000
Available-for-Sale, Gross Unrealized Gains	1,000	1,000
Available-for-Sale, Gross Unrealized Losses	3,000	8,000
Total available-for-sale securities, Fair Value	565,000	565,000
Commercial Mortgage Backed Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	6,187,000	8,568,000
Available-for-Sale, Gross Unrealized Gains	133,000	166,000
Available-for-Sale, Gross Unrealized Losses	0	0
Total available-for-sale securities, Fair Value	6,320,000	8,734,000
Other Structured Financial Products [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	25,038,000	25,038,000
Available-for-Sale, Gross Unrealized Gains	0	0
Available-for-Sale, Gross Unrealized Losses	13,619,000	13,741,000
Total available-for-sale securities, Fair Value	11,419,000	11,297,000
Other Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	43,176,000	43,156,000
Available-for-Sale, Gross Unrealized Gains	2,219,000	1,487,000
Available-for-Sale, Gross Unrealized Losses	569,000	557,000
Total available-for-sale securities, Fair Value	44,826,000	44,086,000
Total Debt Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	2,270,730,000	2,362,893,000
Available-for-Sale, Gross Unrealized Gains	34,985,000	27,546,000
Available-for-Sale, Gross Unrealized Losses	30,730,000	38,907,000
Total available-for-sale securities, Fair Value	2,274,985,000	2,351,532,000
Other Equity Securities [Member]
Schedule Of Available For Sale Securities [Line Items]
Available-for-Sale, Amortized Cost	24,370,000	24,318,000
Available-for-Sale, Gross Unrealized Gains	947,000	557,000
Available-for-Sale, Gross Unrealized Losses	1,067,000	1,183,000
Total available-for-sale securities, Fair Value	$ 24,250,000	$ 23,692,000