Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $) In Thousands, unless otherwise specified	9 Months Ended
	Sep. 30, 2013
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 2,350,297
Held to Maturity, Less than 12 Months, Unrealized Losses	133,504
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	2,350,297
Held to Maturity, Total Unrealized Losses	133,504
Available for Sale, Less than 12 Months, Estimated Fair Value	358,398
Available for Sale, Less than 12 Months, Unrealized Losses	6,130
Available for Sale, More than 12 Months, Estimated Fair Value	46
Available for Sale, More than 12 Months, Unrealized Losses	1
Available for Sale, Total Estimated Fair Value	358,444
Available for Sale, Total Unrealized Losses	6,131
Residential mortgage- backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	7,063
Held to Maturity, Less than 12 Months, Unrealized Losses	57
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	7,063
Held to Maturity, Total Unrealized Losses	57
Available for Sale, Less than 12 Months, Estimated Fair Value	18,244
Available for Sale, Less than 12 Months, Unrealized Losses	1,025
Available for Sale, More than 12 Months, Estimated Fair Value	46
Available for Sale, More than 12 Months, Unrealized Losses	1
Available for Sale, Total Estimated Fair Value	18,290
Available for Sale, Total Unrealized Losses	1,026
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	2,342,235
Held to Maturity, Less than 12 Months, Unrealized Losses	133,446
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	2,342,235
Held to Maturity, Total Unrealized Losses	133,446
Available for Sale, Less than 12 Months, Estimated Fair Value	340,154
Available for Sale, Less than 12 Months, Unrealized Losses	5,105
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	340,154
Available for Sale, Total Unrealized Losses	5,105
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	999
Held to Maturity, Less than 12 Months, Unrealized Losses	1
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	999
Held to Maturity, Total Unrealized Losses	$ 1