Securities - Schedule of Unrealized Losses on Investment Securities (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Other Government Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	$ 55,470	$ 18,894
Less than 12 months, Unrealized Losses	(1,209)	(56)
Less than 12 months, Number of Securities	12	6
12 months or more, Fair Value	4,587
12 months or more, Unrealized Losses	(50)
12 months or more, Number of Securities	1
Fair Value, Total	60,057	18,894
Unrealized Losses, Total	(1,259)	(56)
Number of Securities Total	13	6
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	431,881	38,913
Less than 12 months, Unrealized Losses	(8,469)	(258)
Less than 12 months, Number of Securities	67	20
12 months or more, Fair Value	5,439	9,966
12 months or more, Unrealized Losses	(84)	(126)
12 months or more, Number of Securities	7	10
Fair Value, Total	437,320	48,879
Unrealized Losses, Total	(8,553)	(384)
Number of Securities Total	74	30
Obligations of State and Political Subdivisions [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	187,965	72,521
Less than 12 months, Unrealized Losses	(11,176)	(838)
Less than 12 months, Number of Securities	298	107
12 months or more, Fair Value	6,862
12 months or more, Unrealized Losses	(408)
12 months or more, Number of Securities	5
Fair Value, Total	194,827	72,521
Unrealized Losses, Total	(11,584)	(838)
Number of Securities Total	303	107
Corporate Debt Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	17,981	1,526
Less than 12 months, Unrealized Losses	(410)	(57)
Less than 12 months, Number of Securities	6	2
12 months or more, Fair Value	7,825	10,878
12 months or more, Unrealized Losses	(175)	(177)
12 months or more, Number of Securities	3	5
Fair Value, Total	25,806	12,404
Unrealized Losses, Total	(585)	(234)
Number of Securities Total	9	7
Total Temporarily Impaired Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value	693,297	134,692
Less than 12 months, Unrealized Losses	(21,264)	(1,230)
Less than 12 months, Number of Securities	383	137
12 months or more, Fair Value	24,713	20,844
12 months or more, Unrealized Losses	(717)	(303)
12 months or more, Number of Securities	16	15
Fair Value, Total	718,010	155,536
Unrealized Losses, Total	(21,981)	(1,533)
Number of Securities Total	399	152
Equity Securities [Member]
Schedule Of Available For Sale Securities And Held To Maturity Securities [Line Items]
Less than 12 months, Fair Value		2,838
Less than 12 months, Unrealized Losses		(21)
Less than 12 months, Number of Securities		2
12 months or more, Fair Value
12 months or more, Unrealized Losses
12 months or more, Number of Securities
Fair Value, Total		2,838
Unrealized Losses, Total		$ (21)
Number of Securities Total		2