Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
Mar. 31, 2015
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 227,355
Held to Maturity, Less than 12 Months, Unrealized Losses	1,163
Held to Maturity, More than 12 Months, Estimated Fair Value	296,614
Held to Maturity, More than 12 Months, Unrealized Losses	6,794
Held to Maturity, Total Estimated Fair Value	523,969
Held to Maturity, Total Unrealized Losses	7,957
Available for Sale, Less than 12 Months, Estimated Fair Value	821,095
Available for Sale, Less than 12 Months, Unrealized Losses	5,164
Available for Sale, More than 12 Months, Estimated Fair Value	12,398
Available for Sale, More than 12 Months, Unrealized Losses	282
Available for Sale, Total Estimated Fair Value	833,493
Available for Sale, Total Unrealized Losses	5,446
Residential mortgage-backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	8,360
Available for Sale, Less than 12 Months, Unrealized Losses	95
Available for Sale, More than 12 Months, Estimated Fair Value	12,398
Available for Sale, More than 12 Months, Unrealized Losses	282
Available for Sale, Total Estimated Fair Value	20,758
Available for Sale, Total Unrealized Losses	377
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	227,355
Held to Maturity, Less than 12 Months, Unrealized Losses	1,163
Held to Maturity, More than 12 Months, Estimated Fair Value	296,614
Held to Maturity, More than 12 Months, Unrealized Losses	6,794
Held to Maturity, Total Estimated Fair Value	523,969
Held to Maturity, Total Unrealized Losses	7,957
Available for Sale, Less than 12 Months, Estimated Fair Value	286,513
Available for Sale, Less than 12 Months, Unrealized Losses	3,071
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	286,513
Available for Sale, Total Unrealized Losses	3,071
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	526,222
Available for Sale, Less than 12 Months, Unrealized Losses	1,998
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	526,222
Available for Sale, Total Unrealized Losses	$ 1,998

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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

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