Securities - Schedule of Fair Value and Amortized Cost of Available-for-sale and Held-to-maturity Securities (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	$ 968,848	$ 1,000,505
Available-for-sale, Gross Unrealized Gains	12,028	21,363
Available-for-sale, Gross Unrealized Losses	(9,698)	(624)
Available-for-sale, Estimated Fair Value	971,178	1,021,244
Held-to-maturity, Amortized cost	608,761	602,509
Held-to-maturity, Gross Unrealized Gains	18,725	37,673
Held-to-maturity, Gross Unrealized Losses	(12,283)	(909)
Held-to-maturity securities, Fair value	615,203	639,273
Total securities, Amortized Cost	1,577,609	1,603,014
Total securities, Gross Unrealized Gains	30,753	59,036
Total securities, Gross Unrealized Losses	(21,981)	(1,533)
Total securities, Estimated Fair Value	1,586,381	1,660,517
Other Government Agencies [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	72,082	96,257
Available-for-sale, Gross Unrealized Gains	10	411
Available-for-sale, Gross Unrealized Losses	(1,259)	(56)
Available-for-sale, Estimated Fair Value	70,833	96,612
Residential Mortgage-Backed Securities and Collateralized Mortgage Obligations of Government Agencies [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	717,329	721,824
Available-for-sale, Gross Unrealized Gains	4,857	10,690
Available-for-sale, Gross Unrealized Losses	(7,770)	(304)
Available-for-sale, Estimated Fair Value	714,416	732,210
Held-to-maturity, Amortized cost	117,737	152,872
Held-to-maturity, Gross Unrealized Gains	2,926	6,421
Held-to-maturity, Gross Unrealized Losses	(783)	(80)
Held-to-maturity securities, Fair value	119,880	159,213
Obligations of State and Political Subdivisions [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	126,618	139,511
Available-for-sale, Gross Unrealized Gains	5,838	9,133
Available-for-sale, Gross Unrealized Losses	(242)	(9)
Available-for-sale, Estimated Fair Value	132,214	148,635
Held-to-maturity, Amortized cost	488,313	449,284
Held-to-maturity, Gross Unrealized Gains	15,799	31,244
Held-to-maturity, Gross Unrealized Losses	(11,342)	(829)
Held-to-maturity securities, Fair value	492,770	479,699
Corporate Debt Securities [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	42,886	32,706
Available-for-sale, Gross Unrealized Gains	149	213
Available-for-sale, Gross Unrealized Losses	(427)	(234)
Available-for-sale, Estimated Fair Value	42,608	32,685
Held-to-maturity, Amortized cost	2,711
Held-to-maturity, Gross Unrealized Gains
Held-to-maturity, Gross Unrealized Losses	(158)
Held-to-maturity securities, Fair value	2,553
Debt Securities [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	958,915	990,298
Available-for-sale, Gross Unrealized Gains	10,854	20,447
Available-for-sale, Gross Unrealized Losses	(9,698)	(603)
Available-for-sale, Estimated Fair Value	960,071	1,010,142
Equity Securities [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Available-for-sale, Amortized Cost	9,933	10,207
Available-for-sale, Gross Unrealized Gains	1,174	916
Available-for-sale, Gross Unrealized Losses		(21)
Available-for-sale, Estimated Fair Value	11,107	11,102
Other residential collateralized mortgage obligations [Member]
Investments In Debt And Marketable Equity Securities And Certain Trading Assets [Line Items]
Held-to-maturity, Amortized cost		353
Held-to-maturity, Gross Unrealized Gains		8
Held-to-maturity, Gross Unrealized Losses
Held-to-maturity securities, Fair value		$ 361