Business Segments - Financial Information by Business Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Interest income	$ 54,044	$ 54,424	$ 107,501	$ 109,317
Interest expense	5,737	8,435	11,870	17,199
Net interest income	48,307	45,989	95,631	92,118
Provision for credit losses	849	1,021	3,048	3,123
Net interest income after provision for credit losses	47,458	44,968	92,583	88,995
Non-interest income	18,241	17,724	35,290	35,220
Non-interest expense	40,304	39,499	80,398	80,246
Income before provision for income taxes	25,395	23,193	47,475	43,969
Provision for income taxes	6,520	6,176	12,179	10,932
Net income	18,875	17,017	35,296	33,037
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	54,044	54,424	107,501	109,317
Interest expense	5,737	8,435	11,870	17,199
Net interest income	48,307	45,989	95,631	92,118
Provision for credit losses	849	1,021	3,048	3,123
Net interest income after provision for credit losses	47,458	44,968	92,583	88,995
Non-interest income	13,031	12,901	24,432	25,380
Non-interest expense	37,367	36,620	74,193	74,372
Income before provision for income taxes	23,122	21,249	42,822	40,003
Provision for income taxes	5,611	5,399	10,318	9,346
Net income	17,511	15,850	32,504	30,657
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Interest income
Interest expense
Net interest income
Provision for credit losses
Net interest income after provision for credit losses
Non-interest income	5,210	4,823	10,858	9,840
Non-interest expense	2,937	2,879	6,205	5,874
Income before provision for income taxes	2,273	1,944	4,653	3,966
Provision for income taxes	909	777	1,861	1,586
Net income	$ 1,364	$ 1,167	$ 2,792	$ 2,380