Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2014
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 200,820
Held to Maturity, Less than 12 Months, Unrealized Losses	828
Held to Maturity, More than 12 Months, Estimated Fair Value	1,410,468
Held to Maturity, More than 12 Months, Unrealized Losses	31,457
Held to Maturity, Total Estimated Fair Value	1,611,288
Held to Maturity, Total Unrealized Losses	32,285
Available for Sale, Less than 12 Months, Estimated Fair Value	219,522
Available for Sale, Less than 12 Months, Unrealized Losses	245
Available for Sale, More than 12 Months, Estimated Fair Value	22,887
Available for Sale, More than 12 Months, Unrealized Losses	689
Available for Sale, Total Estimated Fair Value	242,409
Available for Sale, Total Unrealized Losses	934
Residential mortgage- backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	0
Held to Maturity, Less than 12 Months, Unrealized Losses	0
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	0
Held to Maturity, Total Unrealized Losses	0
Available for Sale, Less than 12 Months, Estimated Fair Value	5,384
Available for Sale, Less than 12 Months, Unrealized Losses	12
Available for Sale, More than 12 Months, Estimated Fair Value	17,509
Available for Sale, More than 12 Months, Unrealized Losses	651
Available for Sale, Total Estimated Fair Value	22,893
Available for Sale, Total Unrealized Losses	663
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	200,820
Held to Maturity, Less than 12 Months, Unrealized Losses	828
Held to Maturity, More than 12 Months, Estimated Fair Value	1,410,468
Held to Maturity, More than 12 Months, Unrealized Losses	31,457
Held to Maturity, Total Estimated Fair Value	1,611,288
Held to Maturity, Total Unrealized Losses	32,285
Available for Sale, Less than 12 Months, Estimated Fair Value	63,962
Available for Sale, Less than 12 Months, Unrealized Losses	213
Available for Sale, More than 12 Months, Estimated Fair Value	5,378
Available for Sale, More than 12 Months, Unrealized Losses	38
Available for Sale, Total Estimated Fair Value	69,340
Available for Sale, Total Unrealized Losses	251
U.S. Treasury [Member]
Schedule of Available-for-Sale Securities [Line Items]
Available for Sale, Less than 12 Months, Estimated Fair Value	150,176
Available for Sale, Less than 12 Months, Unrealized Losses	20
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	150,176
Available for Sale, Total Unrealized Losses	$ 20