Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 726,838	$ 18,930
Less Than 12 Months, Unrealized Losses	9,377	58
12 Months or Longer, Fair Value	7,787	5,371
12 Months or Longer, Unrealized Losses	188	24
Total, Fair Value	734,625	24,301
Total, Unrealized Losses	9,565	82
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	147,360	9,994
Less Than 12 Months, Unrealized Losses	1,756	8
12 Months or Longer, Fair Value
12 Months or Longer, Unrealized Losses
Total, Fair Value	147,360	9,994
Total, Unrealized Losses	1,756	8
U.S. Government Agency and Sponsored Enterprises Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	579,478	8,936
Less Than 12 Months, Unrealized Losses	7,621	50
12 Months or Longer, Fair Value	7,787	5,371
12 Months or Longer, Unrealized Losses	188	24
Total, Fair Value	587,265	14,307
Total, Unrealized Losses	$ 7,809	$ 74