Parent Company Condensed Financial Information - Balance Sheets (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Condensed Financial Information [Line Items]
Cash	$ 109,580	$ 108,356	$ 337,711	$ 270,526
Other assets	97,024	103,026
Total Assets	4,964,686	4,533,190
Other liabilities	61,789	48,300
Total Liabilities	4,356,297	3,961,884
Total Shareholders' Equity	608,389	571,306	537,422	490,526
Total Liabilities and Shareholders' Equity	4,964,686	4,533,190
Parent Company [Member]
Condensed Financial Information [Line Items]
Cash	38,028	14,852	12,202	13,684
Bank subsidiary	565,927	553,825
Nonbank subsidiaries	20,569	19,561
Other assets	5,567	4,441
Total Assets	630,091	592,679
Long-term debt	20,619	20,619
Other liabilities	1,083	754
Total Liabilities	21,702	21,373
Total Shareholders' Equity	608,389	571,306
Total Liabilities and Shareholders' Equity	$ 630,091	$ 592,679