Comprehensive Income and Accumulated Other Comprehensive Income - Components of Pretax and Related Income Tax Expense (Benefit) (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Income tax (expense) benefit	$ (3,003)	$ (588)	$ (5,232)	$ (2,460)
Amounts reclassified from accumulated other comprehensive income, net of taxes	(388)		(262)
Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax	597	(63)	403	(1,135)
Income tax (expense) benefit	209	(22)	141	(397)
Amounts reclassified from accumulated other comprehensive income, net of taxes	388	(41)	262	(738)
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, net of taxes	(388)		(262)
Changes in Net Unrealized Gains on Investment Securities Having No Credit Losses Recognized in the Consolidated Statement of Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax	597	(63)	403	(1,136)
Income tax (expense) benefit	209	(22)	141	(397)
Amounts reclassified from accumulated other comprehensive income, net of taxes	388	(41)	262	(739)
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, net of taxes
Changes in Net Unrealized Gains on Investment Securities Having Credit Losses Recognized in the Consolidated Statement of Income [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]
Amounts reclassified from accumulated other comprehensive income, before tax				1
Income tax (expense) benefit
Amounts reclassified from accumulated other comprehensive income, net of taxes				$ 1