Quarterly Financial Data - Summary of Quarterly Financial Data (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 220,506	$ 211,650	$ 211,147	$ 199,116	$ 201,095	$ 191,203	$ 189,028	$ 185,597	$ 842,419	$ 766,923	$ 707,023
Interest expense	48,537	46,796	42,187	36,587	36,700	37,057	37,573	37,444	174,107	148,774	156,754
Net interest income	171,969	164,854	168,960	162,529	164,395	154,146	151,455	148,153	668,312	618,149	550,269
Provision for credit losses	2,200	1,640	3,632	2,470	3,800	5,840	1,429	800	9,942	11,869	8,101
Non-interest income:
Gains on sales of loans, net	6,375	5,520	4,791	4,128	12,307	4,823	3,105	1,795	20,814	22,030	4,245
Other non-interest income	21,229	20,568	19,899	20,931	20,353	20,030	21,159	19,653	16,702	16,953	15,185
Non-interest expense:
Amortization of tax credit investments	20,302	8,389	7,732	5,324	13,384	6,450	7,646	7,264	41,747	34,744	27,312
Other non-interest expense	116,015	124,176	111,507	115,628	111,445	106,818	112,157	110,961	54,922	58,870	55,108
Income Before Income Taxes	61,056	56,737	70,779	64,166	68,426	59,891	54,487	50,576	252,738	233,380	126,895
Income tax expense	34,958	17,088	20,714	18,071	18,336	17,049	15,460	14,389	90,831	65,234	23,938
Net Income	26,098	39,649	50,065	46,095	50,090	42,842	39,027	36,187	161,907	168,146	102,957
Dividends on preferred stock	3,172	2,683	1,797	1,797	1,797	1,797	1,797	1,797	9,449	7,188	3,813
Net Income Available to Common Shareholders	$ 22,926	$ 36,966	$ 48,268	$ 44,298	$ 48,293	$ 41,045	$ 37,230	$ 34,390	$ 152,458	$ 160,958	$ 99,144
Earnings per common share:
Basic (in usd per share)	$ 0.09	$ 0.14	$ 0.18	$ 0.17	$ 0.19	$ 0.16	$ 0.15	$ 0.14	$ 0.58	$ 0.63	$ 0.42
Diluted (in usd per share)	0.09	0.14	0.18	0.17	0.19	0.16	0.15	0.14	0.58	0.63	0.42
Cash dividends declared per common share (in usd per share)	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.11	$ 0.44	$ 0.44	$ 0.44
Weighted average number of common shares outstanding:
Basic (in shares)	264,332,895	264,058,174	263,958,292	263,797,024	256,422,437	254,473,994	254,381,170	254,075,349	264,038,123	254,841,571	234,405,909
Diluted (in shares)	265,288,067	264,936,220	264,778,242	264,546,266	256,952,036	254,940,307	254,771,213	254,347,420	264,889,007	255,268,336	234,437,000