Segment Reporting - Summary of Operating Results of Segments (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net interest income	$ 169,289	$ 91,111
Provision for credit losses	11,210	4,380
Noninterest income	30,664	24,983
Noninterest expense	113,440	61,939
Income tax expense (benefit)	17,102	10,950
Net income	58,201	38,825
Total assets	17,452,911	10,999,382	$ 12,730,285
Banking
Segment Reporting Information [Line Items]
Net interest income	174,396	96,100
Provision for credit losses	11,210	4,380
Noninterest income	20,087	12,438
Noninterest expense	97,324	50,532
Income tax expense (benefit)	19,862	12,446
Net income	66,087	41,181
Total assets	17,347,134	10,903,031
Financial Services
Segment Reporting Information [Line Items]
Net interest income	(630)	(607)
Noninterest income	10,285	12,358
Noninterest expense	7,652	9,978
Income tax expense (benefit)	373	308
Net income	1,630	1,465
Total assets	100,175	91,468
Corporate
Segment Reporting Information [Line Items]
Net interest income	(4,477)	(4,382)
Noninterest income	292	186
Noninterest expense	8,464	1,429
Income tax expense (benefit)	(3,133)	(1,804)
Net income	(9,516)	(3,820)
Total assets	$ 5,602	$ 4,883