Segment Reporting - Financial Information for Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Financial information for the Company's operating segments
Net interest income (loss)	$ 106,286			$ 108,825			$ 318,670	$ 334,772
Provision for credit losses on loans	23,100			1,700			76,350	4,100
Noninterest income	70,928			37,953			172,668	115,798
Noninterest expense	116,510			96,500			349,836	278,622
Income (loss) before income taxes	37,604			48,578			65,152	167,848
Income tax expense (benefit)	7,612			11,132			13,022	38,667
Net income (loss)	29,992	$ 20,130	$ 2,008	37,446	$ 46,625	$ 45,110	52,130	129,181
Total assets	14,808,933			13,039,674			14,808,933	13,039,674	$ 13,400,618
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)	108,909			111,696			325,879	343,418
Provision for credit losses on loans	22,408			1,700			75,481	4,100
Noninterest income	63,918			31,911			152,716	97,789
Noninterest expense	110,430			90,996			332,490	261,905
Income (loss) before income taxes	39,989			50,911			70,624	175,202
Income tax expense (benefit)	8,383			12,009			15,088	41,205
Net income (loss)	31,606			38,902			55,536	133,997
Total assets	14,694,683			12,922,205			14,694,683	12,922,205
Goodwill	936,916			936,916			936,916	936,916
Operating segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	126			177			424	516
Provision for credit losses on loans	0			0			0	0
Noninterest income	2,694			2,533			7,787	7,634
Noninterest expense	1,974			1,948			5,708	5,661
Income (loss) before income taxes	846			762			2,503	2,489
Income tax expense (benefit)	217			200			658	648
Net income (loss)	629			562			1,845	1,841
Total assets	30,138			27,448			30,138	27,448
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)	403			485			1,250	1,244
Provision for credit losses on loans	692			0			869	0
Noninterest income	4,714			3,859			13,370	11,408
Noninterest expense	3,818			3,287			11,215	10,199
Income (loss) before income taxes	607			1,057			2,536	2,453
Income tax expense (benefit)	0			0			0	0
Net income (loss)	607			1,057			2,536	2,453
Total assets	68,261			70,973			68,261	70,973
Goodwill	0			0			0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(3,152)			(3,533)			(8,883)	(10,406)
Provision for credit losses on loans	0			0			0	0
Noninterest income	(398)			(350)			(1,205)	(1,033)
Noninterest expense	288			269			423	857
Income (loss) before income taxes	(3,838)			(4,152)			(10,511)	(12,296)
Income tax expense (benefit)	(988)			(1,077)			(2,724)	(3,186)
Net income (loss)	(2,850)			(3,075)			(7,787)	(9,110)
Total assets	15,851			19,048			15,851	19,048
Goodwill	$ 0			$ 0			$ 0	$ 0