Segment Reporting - Financial Information for Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2021	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Financial information for the Company's operating segments
Net interest income (loss)	$ 103,292			$ 106,286			$ 322,519	$ 318,670
Recovery of credit losses	(1,200)			23,100			(1,200)	76,350
Noninterest income (loss)	50,755			70,928			179,402	172,668
Noninterest expense	103,999			116,510			328,711	349,836
Income (loss) before income taxes	51,248			37,604			174,410	65,152
Income tax expense (benefit)	11,185			7,612			35,572	13,022
Net income (loss)	40,063	$ 40,867	$ 57,908	29,992	$ 20,130	$ 2,008	138,838	52,130
Total assets	16,155,550			14,808,933			16,155,550	14,808,933	$ 14,929,612
Goodwill	939,683			939,683			939,683	939,683	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)	106,506			108,909			332,234	325,879
Recovery of credit losses	(1,088)			22,408			(1,088)	75,481
Noninterest income (loss)	42,546			63,918			155,765	152,716
Noninterest expense	97,437			110,430			309,449	332,490
Income (loss) before income taxes	52,703			39,989			179,638	70,624
Income tax expense (benefit)	12,117			8,383			38,320	15,088
Net income (loss)	40,586			31,606			141,318	55,536
Total assets	16,040,728			14,694,683			16,040,728	14,694,683
Goodwill	936,916			936,916			936,916	936,916
Operating segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	115			126			333	424
Recovery of credit losses	0			0			0	0
Noninterest income (loss)	2,835			2,694			8,558	7,787
Noninterest expense	2,095			1,974			6,005	5,708
Income (loss) before income taxes	855			846			2,886	2,503
Income tax expense (benefit)	220			217			740	658
Net income (loss)	635			629			2,146	1,845
Total assets	33,036			30,138			33,036	30,138
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)	404			403			1,184	1,250
Recovery of credit losses	(112)			692			(112)	869
Noninterest income (loss)	5,820			4,714			16,421	13,370
Noninterest expense	4,189			3,818			12,337	11,215
Income (loss) before income taxes	2,147			607			5,380	2,536
Income tax expense (benefit)	0			0			0	0
Net income (loss)	2,147			607			5,380	2,536
Total assets	67,204			68,261			67,204	68,261
Goodwill	0			0			0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(3,733)			(3,152)			(11,232)	(8,883)
Recovery of credit losses	0			0			0	0
Noninterest income (loss)	(446)			(398)			(1,342)	(1,205)
Noninterest expense	278			288			920	423
Income (loss) before income taxes	(4,457)			(3,838)			(13,494)	(10,511)
Income tax expense (benefit)	(1,152)			(988)			(3,488)	(2,724)
Net income (loss)	(3,305)			(2,850)			(10,006)	(7,787)
Total assets	14,582			15,851			14,582	15,851
Goodwill	$ 0			$ 0			$ 0	$ 0