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, 2015	Dec. 31, 2014	Mar. 31, 2014
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 1,721,030,000	$ 1,792,078,000	$ 1,949,697,000
Trading account securities	7,811,000	1,206,000	1,068,000
Derivative assets	47,550,000	37,841,000	37,239,000
Derivative liabilities	47,698,000	36,921,000	34,990,000
U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	271,474,000	381,805,000	340,178,000
U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	661,793,000	668,316,000	828,275,000
Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	269,912,000	238,529,000	175,300,000
Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	329,591,000	318,710,000	417,303,000
Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	54,446,000	51,139,000	53,574,000
Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	7,811,000	1,206,000	1,068,000
Mortgage loans held-for-sale	446,355,000	351,290,000	215,231,000
Mortgage servicing rights	7,852,000	8,435,000	8,719,000
Nonqualified deferred compensation assets	8,718,000	7,951,000	7,783,000
Derivative assets	62,984,000	47,964,000	51,529,000
Total	2,254,750,000	2,208,924,000	2,234,027,000
Derivative liabilities	55,225,000	41,180,000	37,797,000
Fair Value, Measurements, Recurring [Member] | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	271,474,000	381,805,000	340,178,000
Fair Value, Measurements, Recurring [Member] | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	661,793,000	668,316,000	828,275,000
Fair Value, Measurements, Recurring [Member] | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	269,912,000	238,529,000	175,300,000
Fair Value, Measurements, Recurring [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	133,814,000	133,579,000	135,067,000
Fair Value, Measurements, Recurring [Member] | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	329,591,000	318,710,000	417,303,000
Fair Value, Measurements, Recurring [Member] | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	54,446,000	51,139,000	53,574,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Trading account securities	7,811,000	1,206,000	1,068,000
Mortgage loans held-for-sale	446,355,000	351,290,000	215,231,000
Mortgage servicing rights	0	0	0
Nonqualified deferred compensation assets	8,718,000	7,951,000	7,783,000
Derivative assets	62,984,000	47,964,000	51,529,000
Total	2,166,193,000	2,117,825,000	2,162,098,000
Derivative liabilities	55,225,000	41,180,000	37,797,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	271,474,000	381,805,000	340,178,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	661,793,000	668,316,000	828,275,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	213,863,000	179,576,000	135,528,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	133,814,000	133,579,000	135,067,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	329,591,000	318,710,000	417,303,000
Fair Value, Measurements, Recurring [Member] | Level 2 [Member] | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	29,790,000	27,428,000	30,136,000
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	7,852,000	8,435,000	8,719,000
Nonqualified deferred compensation assets	0	0	0
Derivative assets	0	0	0
Total	88,557,000	91,099,000	71,929,000
Derivative liabilities	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Treasury
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | U.S. Government agencies
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Municipal
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	56,049,000	58,953,000	39,772,000
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Corporate Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Mortgage-backed securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	0	0	0
Fair Value, Measurements, Recurring [Member] | Level 3 [Member] | Equity securities
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities, at fair value	$ 24,656,000	$ 23,711,000	$ 23,438,000