Business Segments - Financial Information by Business Segment (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Segment Reporting Information [Line Items]
Interest income	$ 60,379	$ 53,457	$ 53,457
Interest expense	5,424	6,132	6,132
Net interest income	54,955	47,325	47,325
Provision for credit losses	1,289	2,199	2,199
Net interest income after provision for credit losses	53,666	45,126	45,126
Non-interest income	18,190	17,049	17,049
Non-interest expense	53,441	40,095	40,095
Income before provision for income taxes	18,415	22,080	22,080
Provision for income taxes	4,528	5,659	5,659
Net income	13,887	16,421	16,421
Community Banking [Member]
Segment Reporting Information [Line Items]
Interest income	60,379		53,457
Interest expense	5,424		6,132
Net interest income	54,955		47,325
Provision for credit losses	1,289		2,199
Net interest income after provision for credit losses	53,666		45,126
Non-interest income	12,137		11,401
Non-interest expense	50,278		36,827
Income before provision for income taxes	15,525		19,700
Provision for income taxes	3,372		4,707
Net income	12,153		14,993
Trust and Investment Services [Member]
Segment Reporting Information [Line Items]
Non-interest income	6,053		5,648
Non-interest expense	3,163		3,268
Income before provision for income taxes	2,890		2,380
Provision for income taxes	1,156		952
Net income	$ 1,734		$ 1,428