Securities Available-for-Sale - Continuous Unrealized Loss Position for 12 Months or Less and 12 Months or Longer (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 890,758	$ 127,973
Less than 12 months, Unrealized Losses	24,936	451
12 months or longer, Fair Value	7,659	17,448
12 months or longer, Unrealized Losses	848	820
Total, Fair Value	898,417	145,421
Total, Unrealized Losses	25,784	1,271
U.S. Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	248,324	34,967
Less than 12 months, Unrealized Losses	8,616	68
Total, Fair Value	248,324	34,967
Total, Unrealized Losses	8,616	68
U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	640,699	93,006
Less than 12 months, Unrealized Losses	16,313	383
12 months or longer, Fair Value	2,302	10,169
12 months or longer, Unrealized Losses	18	29
Total, Fair Value	643,001	103,175
Total, Unrealized Losses	16,331	412
Privately Issued Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	1,735
Less than 12 months, Unrealized Losses	7
12 months or longer, Fair Value		1,863
12 months or longer, Unrealized Losses		22
Total, Fair Value	1,735	1,863
Total, Unrealized Losses	7	22
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	3,963	3,963
12 months or longer, Unrealized Losses	724	722
Total, Fair Value	3,963	3,963
Total, Unrealized Losses	724	722
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	1,394	1,453
12 months or longer, Unrealized Losses	106	47
Total, Fair Value	1,394	1,453
Total, Unrealized Losses	106	47
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value
Less than 12 months, Unrealized Losses
12 months or longer, Fair Value
12 months or longer, Unrealized Losses
Total, Fair Value
Total, Unrealized Losses