Quarterly Financial Data - Summarized Quarterly Financial Data (Detail) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Selected Quarterly Financial Information [Abstract]
Net interest income	$ 61,175	$ 61,238	$ 57,159	$ 56,942	$ 55,109	$ 53,973	$ 52,595	$ 54,589	$ 236,514	$ 216,266	$ 236,950
Provision for loan losses	0	(1,000)	(7,600)	(7,500)	(6,800)	(3,750)	(6,200)	0	(16,100)	(16,750)	0
Net earnings	$ 25,581	$ 24,295	$ (25,484)	$ 28,661	$ 25,288	$ 24,239	$ 24,466	$ 21,615	$ 104,021	$ 95,608	$ 77,280
Basic earnings per common share	$ 0.24	$ 0.23	$ 0.24	$ 0.27	$ 0.24	$ 0.23	$ 0.23	$ 0.21	$ 0.98	$ 0.91	$ 0.74
Diluted earnings per common share	$ 0.24	$ 0.23	$ 0.24	$ 0.27	$ 0.24	$ 0.23	$ 0.23	$ 0.21	$ 0.98	$ 0.91	$ 0.74