Summary of Operating Results by Quarter - Summary of Operating Results by Quarter (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 207,292	$ 197,017	$ 192,769	$ 211,411	$ 215,820	$ 219,103	$ 216,662	$ 211,307	$ 808,489	$ 862,892	$ 731,961
Interest expense	12,617	12,633	14,540	37,470	43,457	50,843	50,248	47,439	77,260	191,987	121,515
Net interest income	194,675	184,384	178,229	173,941	172,363	168,260	166,414	163,868	731,229	670,905	610,446
Provision for credit losses	5,000	16,000	21,500	88,000	2,000	7,500	11,000	12,350
Noninterest income	228,290	112,996	120,456	98,424	110,355	103,635	105,398	107,382	560,166	426,770	401,698
Noninterest expense	226,858	197,995	208,533	188,619	203,450	191,397	193,387	190,626	822,005	778,860	717,800
Income tax expense (benefit)	34,787	10,293	8,123	(815)	10,753	10,616	10,466	10,530	52,388	42,365	27,334
Income from continuing operations	$ 156,320	$ 73,092	$ 60,529	$ (3,439)	$ 66,515	$ 62,382	$ 56,959	$ 57,744	$ 286,502	$ 243,600	$ 196,260
Net (loss) income from continuing operations - basic	$ 3.26	$ 1.52	$ 1.26	$ (0.07)	$ 1.36	$ 1.28	$ 1.17	$ 1.19	$ 5.95	$ 4.99	$ 3.98
Net (loss) income from continuing operations - diluted	3.24	1.52	1.26	(0.07)	1.35	1.27	1.16	1.18	5.93	4.96	3.94
Dividend	0.32	0.31	0.31	0.31	0.31	0.30	0.30	0.30	$ 1.25	$ 1.21	$ 1.17
Book value	$ 62.84	$ 59.43	$ 57.84	$ 55.33	$ 53.09	$ 52.23	$ 50.50	$ 47.92