Reportable Segments - Reconciliation of Reportable Segment Revenues, Expenses and Profit (Detail) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Dec. 31, 2019
Segment Reporting Information [Line Items]
Interest income	$ 176,759	$ 132,282
Interest expense	(23,406)	(18,107)
Net interest income	153,353	114,175
Provision for credit losses	(44,914)	(1,053)
Non-interest income	55,790	29,300
Non-interest expense	(122,772)	(84,473)
Net income (loss) before taxes	41,457	57,949
Income tax (provision) benefit	(6,025)	(13,306)
Net income (loss)	35,432	44,643
Total assets	18,596,292	12,587,637	$ 17,142,025
Operating Segments [Member] | Commercial and Retail Banking [Member]
Segment Reporting Information [Line Items]
Interest income	173,702	127,832
Interest expense	(20,434)	(14,851)
Net interest income	153,268	112,981
Provision for credit losses	(45,156)	(1,015)
Non-interest income	27,981	20,300
Non-interest expense	(108,373)	(77,581)
Net income (loss) before taxes	27,720	54,685
Income tax (provision) benefit	(3,487)	(12,690)
Net income (loss)	24,233	41,995
Total assets	17,954,442	11,939,930
Operating Segments [Member] | Correspondent Banking And Capital Markets Division [Member]
Segment Reporting Information [Line Items]
Interest income	3,057	4,450
Interest expense	(1,664)	(2,549)
Net interest income	1,393	1,901
Provision for credit losses	242	(38)
Non-interest income	27,809	9,000
Non-interest expense	(12,503)	(5,713)
Net income (loss) before taxes	16,941	5,150
Income tax (provision) benefit	(4,241)	(1,305)
Net income (loss)	12,700	3,845
Total assets	641,599	647,046
Corporate Overhead and Administration [Member]
Segment Reporting Information [Line Items]
Interest expense	(1,308)	(707)
Net interest income	(1,308)	(707)
Non-interest expense	(1,896)	(1,179)
Net income (loss) before taxes	(3,204)	(1,886)
Income tax (provision) benefit	1,703	689
Net income (loss)	(1,501)	(1,197)
Total assets	3,005,931	2,073,703
Elimination Entries [Member]
Segment Reporting Information [Line Items]
Total assets	$ (3,005,680)	$ (2,073,042)