Fair Values Of Assets And Liabilities (Summary Of Balances Of Assets And Liabilities Measured At Fair Value On A Recurring Basis) (Detail) (USD $)	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2012
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 1,870,831,000	$ 1,796,076,000	$ 1,869,344,000
US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	217,645,000	219,487,000	23,189,000
U.S. Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	973,715,000	990,039,000	682,780,000
Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	110,259,000	110,471,000	69,915,000
Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	368,282,000	271,574,000	886,148,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	1,036,000	583,000	1,140,000
Mortgage loans held-for-sale	370,570,000	385,033,000	339,600,000
Mortgage servicing rights	7,344,000	6,750,000	7,201,000
Nonqualified deferred compensation assets	6,545,000	5,532,000	5,315,000
Derivative assets	52,436,000	53,906,000	39,220,000
Total	2,308,762,000	2,247,880,000	2,261,820,000
Derivative liabilities	54,745,000	57,751,000	46,364,000
Fair Value, Measurements, Recurring [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	217,645,000	219,487,000	23,189,000
Fair Value, Measurements, Recurring [Member] | U.S. Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	973,715,000	990,039,000	682,780,000
Fair Value, Measurements, Recurring [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	110,259,000	110,471,000	69,915,000
Fair Value, Measurements, Recurring [Member] | Corporate Notes And Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	148,873,000	154,806,000	168,816,000
Fair Value, Measurements, Recurring [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	368,282,000	271,574,000	886,148,000
Fair Value, Measurements, Recurring [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	52,057,000	49,699,000	38,496,000
Fair Value, Measurements, Recurring [Member] | Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities
Mortgage loans held-for-sale
Mortgage servicing rights
Nonqualified deferred compensation assets
Derivative assets
Total
Derivative liabilities
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | U.S. Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Corporate Notes And Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Level 1 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	1,036,000	583,000	1,140,000
Mortgage loans held-for-sale	370,570,000	385,033,000	339,600,000
Mortgage servicing rights	0	0	0
Nonqualified deferred compensation assets	6,545,000	5,532,000	5,315,000
Derivative assets	52,436,000	53,906,000	39,220,000
Total	2,244,624,000	2,188,191,000	2,207,860,000
Derivative liabilities	54,745,000	57,751,000	46,364,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	217,645,000	219,487,000	23,189,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	973,715,000	990,039,000	682,780,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	77,935,000	79,701,000	44,380,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Notes And Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	148,873,000	154,806,000	168,816,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	368,282,000	271,574,000	886,148,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	27,587,000	27,530,000	17,272,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	0	0	0
Mortgage loans held-for-sale	0	0	0
Mortgage servicing rights		6,750,000	7,201,000
Nonqualified deferred compensation assets	0	0	0
Derivative assets	0	0	0
Total	64,138,000	59,689,000	53,960,000
Derivative liabilities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | US Treasury Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Government Agencies Debt Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	32,324,000	30,770,000	25,535,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate Notes And Other [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Collateralized Mortgage Backed Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	$ 24,470,000	$ 22,169,000	$ 21,224,000