Securities - Securities, with Unrealized Losses Segregated by Length of Impairment (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Jun. 30, 2013
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	$ 2,433,051
Held to Maturity, Less than 12 Months, Unrealized Losses	127,954
Held to Maturity, More than 12 Months, Estimated Fair Value
Held to Maturity, More than 12 Months, Unrealized Losses
Held to Maturity, Total Estimated Fair Value	2,433,051
Held to Maturity, Total Unrealized Losses	127,954
Available for Sale, Less than 12 Months, Estimated Fair Value	426,618
Available for Sale, Less than 12 Months, Unrealized Losses	7,194
Available for Sale, More than 12 Months, Estimated Fair Value
Available for Sale, More than 12 Months, Unrealized Losses
Available for Sale, Total Estimated Fair Value	426,618
Available for Sale, Total Unrealized Losses	7,194
Residential mortgage- backed securities [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	7,270
Held to Maturity, Less than 12 Months, Unrealized Losses	109
Held to Maturity, More than 12 Months, Estimated Fair Value
Held to Maturity, More than 12 Months, Unrealized Losses
Held to Maturity, Total Estimated Fair Value	7,270
Held to Maturity, Total Unrealized Losses	109
Available for Sale, Less than 12 Months, Estimated Fair Value	87,714
Available for Sale, Less than 12 Months, Unrealized Losses	1,089
Available for Sale, More than 12 Months, Estimated Fair Value
Available for Sale, More than 12 Months, Unrealized Losses
Available for Sale, Total Estimated Fair Value	87,714
Available for Sale, Total Unrealized Losses	1,089
States and political subdivisions [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	2,424,783
Held to Maturity, Less than 12 Months, Unrealized Losses	127,843
Held to Maturity, More than 12 Months, Estimated Fair Value
Held to Maturity, More than 12 Months, Unrealized Losses
Held to Maturity, Total Estimated Fair Value	2,424,783
Held to Maturity, Total Unrealized Losses	127,843
Available for Sale, Less than 12 Months, Estimated Fair Value	338,904
Available for Sale, Less than 12 Months, Unrealized Losses	6,105
Available for Sale, More than 12 Months, Estimated Fair Value
Available for Sale, More than 12 Months, Unrealized Losses
Available for Sale, Total Estimated Fair Value	338,904
Available for Sale, Total Unrealized Losses	6,105
Other [Member]
Schedule of Available-for-Sale Securities [Line Items]
Held to Maturity, Less than 12 Months, Estimated Fair Value	998
Held to Maturity, Less than 12 Months, Unrealized Losses	2
Held to Maturity, More than 12 Months, Estimated Fair Value
Held to Maturity, More than 12 Months, Unrealized Losses
Held to Maturity, Total Estimated Fair Value	998
Held to Maturity, Total Unrealized Losses	$ 2