Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) - Jun. 30, 2015 - USD ($) $ in Thousands	Total
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 850,768
Held to Maturity, Less than 12 Months, Unrealized Losses	11,136
Held to Maturity, More than 12 Months, Estimated Fair Value	251,968
Held to Maturity, More than 12 Months, Unrealized Losses	7,463
Held to Maturity, Total Estimated Fair Value	1,102,736
Held to Maturity, Total Unrealized Losses	18,599
Available for Sale, Less than 12 Months, Estimated Fair Value	1,921,295
Available for Sale, Less than 12 Months, Unrealized Losses	33,690
Available for Sale, More than 12 Months, Estimated Fair Value	11,832
Available for Sale, More than 12 Months, Unrealized Losses	480
Available for Sale, Total Estimated Fair Value	1,933,127
Available for Sale, Total Unrealized Losses	34,170
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	5,123
Held to Maturity, Less than 12 Months, Unrealized Losses	10
Held to Maturity, More than 12 Months, Estimated Fair Value	0
Held to Maturity, More than 12 Months, Unrealized Losses	0
Held to Maturity, Total Estimated Fair Value	5,123
Held to Maturity, Total Unrealized Losses	10
Available for Sale, Less than 12 Months, Estimated Fair Value	32,437
Available for Sale, Less than 12 Months, Unrealized Losses	393
Available for Sale, More than 12 Months, Estimated Fair Value	11,832
Available for Sale, More than 12 Months, Unrealized Losses	480
Available for Sale, Total Estimated Fair Value	44,269
Available for Sale, Total Unrealized Losses	873
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	845,645
Held to Maturity, Less than 12 Months, Unrealized Losses	11,126
Held to Maturity, More than 12 Months, Estimated Fair Value	251,968
Held to Maturity, More than 12 Months, Unrealized Losses	7,463
Held to Maturity, Total Estimated Fair Value	1,097,613
Held to Maturity, Total Unrealized Losses	18,589
Available for Sale, Less than 12 Months, Estimated Fair Value	1,365,860
Available for Sale, Less than 12 Months, Unrealized Losses	28,503
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	1,365,860
Available for Sale, Total Unrealized Losses	28,503
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	522,998
Available for Sale, Less than 12 Months, Unrealized Losses	4,794
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	522,998
Available for Sale, Total Unrealized Losses	$ 4,794

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous loss position for twelve months or longer.

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-Publisher FASB

-Name Accounting Standards Codification

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