Securities Available-for-Sale - Summary of Securities Available-for-Sale (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	$ 1,568,936	$ 1,414,595
Gross Unrealized Gains	9,064	21,477
Gross Unrealized Losses	25,784	1,271
Total, Fair Value	1,552,216	1,434,801
U.S. Treasury [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,996	2,000
Gross Unrealized Gains		4
Gross Unrealized Losses	4
Total, Fair Value	1,992	2,004
U.S. Government Sponsored Enterprises [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	254,944	130,048
Gross Unrealized Gains		360
Gross Unrealized Losses	8,612	68
Total, Fair Value	246,332	130,340
SBA Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	7,760	8,043
Gross Unrealized Gains	107	113
Total, Fair Value	7,867	8,156
U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	1,244,471	1,212,953
Gross Unrealized Gains	8,774	20,816
Gross Unrealized Losses	16,331	412
Total, Fair Value	1,236,914	1,233,357
Privately Issued Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,651	2,938
Gross Unrealized Gains	9	31
Gross Unrealized Losses	7	22
Total, Fair Value	2,653	2,947
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	54,367	55,855
Gross Unrealized Gains	25	41
Gross Unrealized Losses	724	722
Total, Fair Value	53,668	55,174
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	2,300	2,300
Gross Unrealized Losses	106	47
Total, Fair Value	2,194	2,253
Equity Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Amortized cost	447	458
Gross Unrealized Gains	149	112
Total, Fair Value	$ 596	$ 570