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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Quarterly Financial Data [Abstract]
Interest income	$ 176,553	$ 185,853	$ 189,063	$ 178,918	$ 175,234	$ 155,369	$ 153,736	$ 147,706	$ 730,387	$ 632,045	$ 495,336
Interest expense	27,654	32,757	33,833	31,870	29,009	24,527	21,773	19,134	126,114	94,443	58,168
Net interest income	148,899	153,096	155,230	147,048	146,225	130,842	131,963	128,572	604,273	537,602	437,168
Provision for loan losses	1,264	1,437	1,003	1,043	3,390	750	2,446	380	4,747	6,966	3,050
Noninterest income	47,726	53,961	51,214	46,416	58,154	45,957	49,289	41,905	199,317	195,305	183,382
Noninterest expense	134,743	122,585	128,118	123,041	150,268	119,376	130,460	117,157	508,487	517,261	448,836
Income before income taxes	60,618	83,035	77,323	69,380	50,721	56,673	48,346	52,940	290,356	208,680	168,664
Income tax expense	11,433	13,254	14,359	13,104	3,223	5,325	4,345	4,957	52,150	17,850	72,939
Net income	$ 49,185	$ 69,781	$ 62,964	$ 56,276	$ 47,498	$ 51,348	$ 44,001	$ 47,983	$ 238,206	$ 190,830	$ 95,725
Net income (loss) per share, Basic	$ 0.29	$ 0.41	$ 0.37	$ 0.32	$ 0.28	$ 0.34	$ 0.29	$ 0.32	$ 1.39	$ 1.23	$ 0.69
Net income (loss) per share, Diluted	$ 0.29	$ 0.41	$ 0.36	$ 0.32	$ 0.28	$ 0.34	$ 0.29	$ 0.31	$ 1.38	$ 1.22	$ 0.69
Average shares, Basic	169,235	170,746	172,985	174,734	167,044	151,930	151,878	151,721	171,907	155,675	137,821
Average shares, Diluted	170,186	171,551	173,675	175,368	167,992	152,784	152,568	152,370	172,687	156,539	138,513