Segment Reporting - Financial Information for Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended				6 Months Ended
	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2020	Jun. 30, 2019	Dec. 31, 2019
Financial information for the Company's operating segments
Net interest income (loss)	$ 105,782		$ 112,800		$ 212,384	$ 225,947
Provision for credit losses on loans	26,900		900		53,250	2,400
Noninterest income	64,170		41,960		101,740	77,845
Noninterest expense	118,285		93,290		233,326	182,122
Income (loss) before income taxes	24,767		60,570		27,548	119,270
Income tax expense (benefit)	4,637		13,945		5,410	27,535
Net income (loss)	20,130	$ 2,008	46,625	$ 45,110	22,138	91,735
Total assets	14,897,207		12,892,653		14,897,207	12,892,653	$ 13,400,618
Goodwill	939,683		932,971		939,683	932,971	939,683
Community Banks
Financial information for the Company's operating segments
Goodwill	936,916				936,916		936,916
Insurance
Financial information for the Company's operating segments
Goodwill	2,767				2,767		$ 2,767
Operating segments | Community Banks
Financial information for the Company's operating segments
Net interest income (loss)	108,110		115,664		216,970	231,722
Provision for credit losses on loans	26,861		900		53,073	2,400
Noninterest income	58,115		36,293		88,798	65,878
Noninterest expense	112,776		87,596		222,060	170,909
Income (loss) before income taxes	26,588		63,461		30,635	124,291
Income tax expense (benefit)	5,425		14,910		6,705	29,196
Net income (loss)	21,163		48,551		23,930	95,095
Total assets	14,775,811		12,790,623		14,775,811	12,790,623
Goodwill	936,916		930,204		936,916	930,204
Operating segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	111		171		298	339
Provision for credit losses on loans	0		0		0	0
Noninterest income	2,153		2,222		5,093	5,101
Noninterest expense	1,848		1,898		3,734	3,713
Income (loss) before income taxes	416		495		1,657	1,727
Income tax expense (benefit)	111		128		441	448
Net income (loss)	305		367		1,216	1,279
Total assets	29,095		26,722		29,095	26,722
Goodwill	2,767		2,767		2,767	2,767
Operating segments | Wealth Management
Financial information for the Company's operating segments
Net interest income (loss)	397		409		847	759
Provision for credit losses on loans	39		0		177	0
Noninterest income	4,312		3,890		8,656	7,549
Noninterest expense	3,452		3,464		7,397	6,912
Income (loss) before income taxes	1,218		835		1,929	1,396
Income tax expense (benefit)	0		0		0	0
Net income (loss)	1,218		835		1,929	1,396
Total assets	68,257		61,363		68,257	61,363
Goodwill	0		0		0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(2,836)		(3,444)		(5,731)	(6,873)
Provision for credit losses on loans	0		0		0	0
Noninterest income	(410)		(445)		(807)	(683)
Noninterest expense	209		332		135	588
Income (loss) before income taxes	(3,455)		(4,221)		(6,673)	(8,144)
Income tax expense (benefit)	(899)		(1,093)		(1,736)	(2,109)
Net income (loss)	(2,556)		(3,128)		(4,937)	(6,035)
Total assets	24,044		13,945		24,044	13,945
Goodwill	$ 0		$ 0		$ 0	$ 0