Segment Reporting - Financial Information for Operating Segments (Details) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020
Financial information for the Company's operating segments
Net interest income (loss)	$ 109,579,000		$ 105,782,000		$ 219,227,000	$ 212,384,000
Provision for credit losses	0		26,900,000		0	53,250,000
Noninterest income (loss)	47,610,000		64,170,000		128,647,000	101,740,000
Noninterest expense	108,777,000		118,285,000		224,712,000	233,326,000
Income (loss) before income taxes	48,412,000		24,767,000		123,162,000	27,548,000
Income tax expense (benefit)	7,545,000		4,637,000		24,387,000	5,410,000
Net income (loss)	40,867,000	$ 57,908,000	20,130,000	$ 2,008,000	98,775,000	22,138,000
Total assets	16,022,386,000		14,897,207,000		16,022,386,000	14,897,207,000	$ 14,929,612,000
Goodwill	939,683,000		939,683,000		939,683,000	939,683,000	939,683,000
Community Banks
Financial information for the Company's operating segments
Goodwill	936,916,000				936,916,000		936,916,000
Insurance
Financial information for the Company's operating segments
Goodwill	2,767,000				2,767,000		$ 2,767,000
Operating segments | Community Banks
Financial information for the Company's operating segments
Net interest income (loss)	112,780,000		108,110,000		225,728,000	216,970,000
Provision for credit losses	0		26,861,000		0	53,073,000
Noninterest income (loss)	40,149,000		58,115,000		113,219,000	88,798,000
Noninterest expense	102,426,000		112,776,000		212,012,000	222,060,000
Income (loss) before income taxes	50,503,000		26,588,000		126,935,000	30,635,000
Income tax expense (benefit)	8,547,000		5,425,000		26,203,000	6,705,000
Net income (loss)	41,956,000		21,163,000		100,732,000	23,930,000
Total assets	15,910,027,000		14,775,811,000		15,910,027,000	14,775,811,000
Goodwill	936,916,000		936,916,000		936,916,000	936,916,000
Operating segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	111,000		111,000		218,000	298,000
Provision for credit losses	0		0		0	0
Noninterest income (loss)	2,475,000		2,153,000		5,723,000	5,093,000
Noninterest expense	1,987,000		1,848,000		3,910,000	3,734,000
Income (loss) before income taxes	599,000		416,000		2,031,000	1,657,000
Income tax expense (benefit)	153,000		111,000		520,000	441,000
Net income (loss)	446,000		305,000		1,511,000	1,216,000
Total assets	31,978,000		29,095,000		31,978,000	29,095,000
Goodwill	2,767,000		2,767,000		2,767,000	2,767,000
Operating segments | Wealth Management
Financial information for the Company's operating segments
Net interest income (loss)	396,000		397,000		780,000	847,000
Provision for credit losses	0		39,000		0	177,000
Noninterest income (loss)	5,430,000		4,312,000		10,601,000	8,656,000
Noninterest expense	4,047,000		3,452,000		8,148,000	7,397,000
Income (loss) before income taxes	1,779,000		1,218,000		3,233,000	1,929,000
Income tax expense (benefit)	0		0		0	0
Net income (loss)	1,779,000		1,218,000		3,233,000	1,929,000
Total assets	63,656,000		68,257,000		63,656,000	68,257,000
Goodwill	0		0		0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(3,708,000)		(2,836,000)		(7,499,000)	(5,731,000)
Provision for credit losses	0		0		0	0
Noninterest income (loss)	(444,000)		(410,000)		(896,000)	(807,000)
Noninterest expense	317,000		209,000		642,000	135,000
Income (loss) before income taxes	(4,469,000)		(3,455,000)		(9,037,000)	(6,673,000)
Income tax expense (benefit)	(1,155,000)		(899,000)		(2,336,000)	(1,736,000)
Net income (loss)	(3,314,000)		(2,556,000)		(6,701,000)	(4,937,000)
Total assets	16,725,000		24,044,000		16,725,000	24,044,000
Goodwill	$ 0		$ 0		$ 0	$ 0