Reconciliation Between Income Tax Expense and Amount Computed by Applying Federal Statutory Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reconciliation of Unrecognized Tax Benefits [Line Items]
Statutory federal income tax expense									$ 59,578	$ 51,226	$ 44,398
Tax-exempt interest income									(13,480)	(12,301)	(10,365)
State and local income taxes, net of federal tax benefits									2,475	1,193	431
Federal tax credits									(1,090)	(687)	(564)
Other									24	456	1,949
Total tax expense	$ (6,484)	$ (10,156)	$ (12,248)	$ (18,619)	$ (6,815)	$ (10,088)	$ (10,272)	$ (12,712)	$ 47,507	$ 39,887	$ 35,849