Investment Securities Held-to-Maturity - Unrealized Market Loss of Securities (Detail) - USD ($) $ in Thousands	Sep. 30, 2015	Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	$ 164,251	$ 216,533
Less Than 12 Months, Unrealized Losses	796	1,703
12 Months or Longer, Fair Value	112,323	323,302
12 Months or Longer, Unrealized Losses	1,406	5,378
Total, Fair Value	276,574	539,835
Total, Unrealized Losses	2,202	7,081
U.S. Government Sponsored Enterprises [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value		22,414
Less Than 12 Months, Unrealized Losses		25
12 Months or Longer, Fair Value		14,776
12 Months or Longer, Unrealized Losses		224
Total, Fair Value		37,190
Total, Unrealized Losses		249
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less Than 12 Months, Fair Value	164,251	194,119
Less Than 12 Months, Unrealized Losses	796	1,678
12 Months or Longer, Fair Value	112,323	308,526
12 Months or Longer, Unrealized Losses	1,406	5,154
Total, Fair Value	276,574	502,645
Total, Unrealized Losses	$ 2,202	$ 6,832