Interim Financial Data (Unaudited) - Interim Financial Data (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ 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 Data [Abstract]
Interest income	$ 135,134	$ 123,525	$ 118,209	$ 118,468	$ 121,849	$ 119,713	$ 110,243	$ 95,329	$ 495,336	$ 447,134	$ 399,189
Interest expense	16,578	15,047	13,876	12,667	11,932	11,910	10,903	9,686	58,168	44,431	33,073
Net interest income	118,556	108,478	104,333	105,801	109,917	107,803	99,340	85,643	437,168	402,703	366,116
Provision for loan losses	1,037	311	1,355	347	(1,756)	1,306	1,319	91	3,050	960	2,923
Noninterest income	44,825	46,366	49,271	42,920	62,751	47,243	93,385	49,451	183,382	252,830	230,632
Noninterest expense	140,432	103,702	102,811	101,891	126,258	108,062	121,472	98,355	448,836	454,147	430,932
Income before income taxes	21,912	50,831	49,438	46,483	48,166	45,678	69,934	36,648	168,664	200,426	162,893
Income tax expense	40,405	11,459	10,584	10,491	14,710	10,969	30,812	9,671	72,939	66,162	46,177
Net income (loss)	$ (18,493)	$ 39,372	$ 38,854	$ 35,992	$ 33,456	$ 34,709	$ 39,122	$ 26,977	$ 95,725	$ 134,264	$ 116,716
Net income per share, Basic	$ (0.13)	$ 0.30	$ 0.28	$ 0.27	$ 0.25	$ 0.25	$ 0.31	$ 0.24	$ 0.69	$ 1.05	$ 1.01
Net income per share, Diluted	$ (0.13)	$ 0.29	$ 0.28	$ 0.27	$ 0.25	$ 0.25	$ 0.31	$ 0.24	$ 0.69	$ 1.05	$ 1.00
Average shares, Basic	146,073	135,120	135,085	134,912	134,670	134,492	127,508	113,998	137,821	127,705	115,726
Average shares, Diluted	146,875	135,796	135,697	135,431	135,383	135,011	127,973	114,563	138,513	128,301	116,255