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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Data [Abstract]
Interest income	$ 175,234	$ 155,369	$ 153,736	$ 147,706	$ 135,134	$ 123,525	$ 118,209	$ 118,468	$ 632,045	$ 495,336	$ 447,134
Interest expense	29,009	24,527	21,773	19,134	16,578	15,047	13,876	12,667	94,443	58,168	44,431
Net interest income	146,225	130,842	131,963	128,572	118,556	108,478	104,333	105,801	537,602	437,168	402,703
Provision for loan losses	3,390	750	2,446	380	1,037	311	1,355	347	6,966	3,050	960
Noninterest income	58,154	45,957	49,289	41,905	44,825	46,366	49,271	42,920	195,305	183,382	252,830
Noninterest expense	150,268	119,376	130,460	117,157	140,432	103,702	102,811	101,891	517,261	448,836	454,147
Income before income taxes	50,721	56,673	48,346	52,940	21,912	50,831	49,438	46,483	208,680	168,664	200,426
Income tax expense	3,223	5,325	4,345	4,957	40,405	11,459	10,584	10,491	17,850	72,939	66,162
Net income	$ 47,498	$ 51,348	$ 44,001	$ 47,983	$ (18,493)	$ 39,372	$ 38,854	$ 35,992	$ 190,830	$ 95,725	$ 134,264
Net income (loss) per share, Basic	$ 0.28	$ 0.34	$ 0.29	$ 0.32	$ (0.13)	$ 0.30	$ 0.28	$ 0.27	$ 1.23	$ 0.69	$ 1.05
Net income (loss) per share, Dulited	$ 0.28	$ 0.34	$ 0.29	$ 0.31	$ (0.13)	$ 0.29	$ 0.28	$ 0.27	$ 1.22	$ 0.69	$ 1.05
Average shares, Basic	167,044	151,930	151,878	151,721	146,073	135,120	135,085	134,912	155,675	137,821	127,705
Average shares, Diluted	167,992	152,784	152,568	152,370	146,875	135,796	135,697	135,431	156,539	138,513	128,301