Securities (Year End Securities with Unrealized Losses Segregated by Length of Impairment) (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 68,024	$ 2,078,455
Held to Maturity, Less than 12 Months, Unrealized Losses	144	113,655
Held to Maturity, More than 12 Months, Estimated Fair Value	683,251	266,566
Held to Maturity, More than 12 Months, Unrealized Losses	8,891	24,349
Held to Maturity, Total Estimated Fair Value	751,275	2,345,021
Held to Maturity, Total Unrealized Losses	9,035	138,004
Available for Sale, Less than 12 Months, Estimated Fair Value	1,093,531	766,904
Available for Sale, Less than 12 Months, Unrealized Losses	3,439	8,693
Available for Sale, More than 12 Months, Estimated Fair Value	16,944	2,713
Available for Sale, More than 12 Months, Unrealized Losses	375	282
Available for Sale, Total Estimated Fair Value	1,110,475	769,617
Available for Sale, Total Unrealized Losses	3,814	8,975
States and Political Subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	68,024	2,071,521
Held to Maturity, Less than 12 Months, Unrealized Losses	144	113,512
Held to Maturity, More than 12 Months, Estimated Fair Value	683,251	266,566
Held to Maturity, More than 12 Months, Unrealized Losses	8,891	24,349
Held to Maturity, Total Estimated Fair Value	751,275	2,338,087
Held to Maturity, Total Unrealized Losses	9,035	137,861
Available for Sale, Less than 12 Months, Estimated Fair Value	65,751	679,923
Available for Sale, Less than 12 Months, Unrealized Losses	156	7,492
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	65,751	679,923
Available for Sale, Total Unrealized Losses	156	7,492
US Treasury Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	1,019,230
Available for Sale, Less than 12 Months, Unrealized Losses	3,241
Available for Sale, More than 12 Months, Estimated Fair Value	0
Available for Sale, More than 12 Months, Unrealized Losses	0
Available for Sale, Total Estimated Fair Value	1,019,230
Available for Sale, Total Unrealized Losses	3,241
US Government Corporations and Agencies Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value		53,980
Available for Sale, Less than 12 Months, Unrealized Losses		44
Available for Sale, Total Estimated Fair Value		53,980
Available for Sale, Total Unrealized Losses		44
Residential Mortgage Backed Securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value		6,934
Held to Maturity, Less than 12 Months, Unrealized Losses		143
Held to Maturity, Total Estimated Fair Value		6,934
Held to Maturity, Total Unrealized Losses		143
Available for Sale, Less than 12 Months, Estimated Fair Value	8,550	33,001
Available for Sale, Less than 12 Months, Unrealized Losses	42	1,157
Available for Sale, More than 12 Months, Estimated Fair Value	16,944	2,713
Available for Sale, More than 12 Months, Unrealized Losses	375	282
Available for Sale, Total Estimated Fair Value	25,494	35,714
Available for Sale, Total Unrealized Losses	$ 417	$ 1,439

Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer.

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-Name Accounting Standards Codification

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