Fair Value Disclosures - Financial Assets and Financial Liabilities Measured at Fair Value on Recurring Basis (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	$ 1,958,151
U.S. Treasury securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	6,039
Obligations of U.S. government sponsored-enterprises and agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	193,364
Obligations of states and political subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	917,915
Corporate bonds and other [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	113,067
Residential mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	616,566
Commercial mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	107,045
Other securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,155
Level 1 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	161,959
Level 1 Inputs [Member] | U.S. Treasury securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	6,039
Level 1 Inputs [Member] | Obligations of U.S. government sponsored-enterprises and agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	16,844
Level 1 Inputs [Member] | Obligations of states and political subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	86,393
Level 1 Inputs [Member] | Corporate bonds and other [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 1 Inputs [Member] | Residential mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	37,972
Level 1 Inputs [Member] | Commercial mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	10,556
Level 1 Inputs [Member] | Other securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	4,155
Level 2 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	1,796,192
Level 2 Inputs [Member] | U.S. Treasury securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 2 Inputs [Member] | Obligations of U.S. government sponsored-enterprises and agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	176,520
Level 2 Inputs [Member] | Obligations of states and political subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	831,522
Level 2 Inputs [Member] | Corporate bonds and other [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	113,067
Level 2 Inputs [Member] | Residential mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	578,594
Level 2 Inputs [Member] | Commercial mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value	96,489
Level 2 Inputs [Member] | Other securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | U.S. Treasury securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | Obligations of U.S. government sponsored-enterprises and agencies [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | Obligations of states and political subdivisions [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | Corporate bonds and other [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | Residential mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | Commercial mortgage-backed securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value
Level 3 Inputs [Member] | Other securities [Member]
Available-for-sale investment securities:
Available-for-sale investment securities, Total Fair Value