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	Dec. 31, 2014	Dec. 31, 2013
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	$ 24,457	$ 291,195
Less than 12 months, Unrealized Losses	85	1,375
12 months or longer, Fair Value	83,483	29,753
12 months or longer, Unrealized Losses	1,365	1,144
Total, Fair Value	107,940	320,948
Total, Unrealized Losses	1,450	2,519
U.S. Government Agency and Sponsored Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value	24,457	289,709
Less than 12 months, Unrealized Losses	85	1,352
12 months or longer, Fair Value	77,585	24,557
12 months or longer, Unrealized Losses	367	149
Total, Fair Value	102,042	314,266
Total, Unrealized Losses	452	1,501
Privately Issued Residential Mortgage Backed Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
Less than 12 months, Fair Value		1,486
Less than 12 months, Unrealized Losses		23
12 months or longer, Fair Value	678
12 months or longer, Unrealized Losses	25
Total, Fair Value	678	1,486
Total, Unrealized Losses	25	23
Obligations Issued by States and Political Subdivisions [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	3,820	3,820
12 months or longer, Unrealized Losses	873	870
Total, Fair Value	3,820	3,820
Total, Unrealized Losses	873	870
Other Debt Securities [Member]
Schedule of Available-for-sale Securities [Line Items]
12 months or longer, Fair Value	1,400	1,376
12 months or longer, Unrealized Losses	100	125
Total, Fair Value	1,400	1,376
Total, Unrealized Losses	$ 100	$ 125