Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2014
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 90,488
Held to Maturity, Less than 12 Months, Unrealized Losses	297
Held to Maturity, More than 12 Months, Estimated Fair Value	985,618
Held to Maturity, More than 12 Months, Unrealized Losses	15,663
Held to Maturity, Total Estimated Fair Value	1,076,106
Held to Maturity, Total Unrealized Losses	15,960
Available for Sale, Less than 12 Months, Estimated Fair Value	254,903
Available for Sale, Less than 12 Months, Unrealized Losses	943
Available for Sale, More than 12 Months, Estimated Fair Value	17,105
Available for Sale, More than 12 Months, Unrealized Losses	607
Available for Sale, Total Estimated Fair Value	272,008
Available for Sale, Total Unrealized Losses	1,550
Residential mortgage- backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	3,242
Held to Maturity, Less than 12 Months, Unrealized Losses	20
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	3,242
Held to Maturity, Total Unrealized Losses	20
Available for Sale, Less than 12 Months, Estimated Fair Value	7,423
Available for Sale, Less than 12 Months, Unrealized Losses	54
Available for Sale, More than 12 Months, Estimated Fair Value	17,105
Available for Sale, More than 12 Months, Unrealized Losses	607
Available for Sale, Total Estimated Fair Value	24,528
Available for Sale, Total Unrealized Losses	661
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	87,246
Held to Maturity, Less than 12 Months, Unrealized Losses	277
Held to Maturity, More than 12 Months, Estimated Fair Value	985,618
Held to Maturity, More than 12 Months, Unrealized Losses	15,663
Held to Maturity, Total Estimated Fair Value	1,072,864
Held to Maturity, Total Unrealized Losses	15,940
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	247,480
Available for Sale, Less than 12 Months, Unrealized Losses	889
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	247,480
Available for Sale, Total Unrealized Losses	$ 889