Segment Reporting - Financial Information for Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Financial information for the Company's operating segments
Net interest income (loss)	$ 108,825			$ 99,439			$ 334,772	$ 281,068
Provision for loan losses	1,700			2,250			4,100	5,810
Noninterest income	37,953			38,053			115,798	107,587
Noninterest expense	96,500			94,746			278,622	251,716
Income before income taxes	48,578			40,496			167,848	131,129
Income tax expense (benefit)	11,132			8,532			38,667	28,629
Net income (loss)	37,446	$ 46,625	$ 45,110	31,964	$ 36,710	$ 33,826	129,181	102,500
Total assets	13,039,674			12,746,939			13,039,674	12,746,939	$ 12,934,878
Goodwill	939,683			927,261			939,683	927,261	932,928
Community Banks
Financial information for the Company's operating segments
Goodwill	936,916						936,916		930,161
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)	111,696			101,970			343,418	288,073
Provision for loan losses	1,700			2,250			4,100	5,810
Noninterest income	31,911			32,140			97,789	90,007
Noninterest expense	90,996			89,370			261,905	235,631
Income before income taxes	50,911			42,490			175,202	136,639
Income tax expense (benefit)	12,009			9,226			41,205	30,558
Net income (loss)	38,902			33,264			133,997	106,081
Total assets	12,922,205			12,634,614			12,922,205	12,634,614
Goodwill	936,916			924,494			936,916	924,494
Operating Segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	177			124			516	348
Provision for loan losses	0			0			0	0
Noninterest income	2,533			2,488			7,634	7,408
Noninterest expense	1,948			1,899			5,661	5,449
Income before income taxes	762			713			2,489	2,307
Income tax expense (benefit)	200			186			648	599
Net income (loss)	562			527			1,841	1,708
Total assets	27,448			25,236			27,448	25,236
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)	485			324			1,244	952
Provision for loan losses	0			0			0	0
Noninterest income	3,859			3,641			11,408	10,882
Noninterest expense	3,287			3,284			10,199	9,889
Income before income taxes	1,057			681			2,453	1,945
Income tax expense (benefit)	0			0			0	0
Net income (loss)	1,057			681			2,453	1,945
Total assets	70,973			62,502			70,973	62,502
Goodwill	0			0			0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(3,533)			(2,979)			(10,406)	(8,305)
Provision for loan losses	0			0			0	0
Noninterest income	(350)			(216)			(1,033)	(710)
Noninterest expense	269			193			857	747
Income before income taxes	(4,152)			(3,388)			(12,296)	(9,762)
Income tax expense (benefit)	(1,077)			(880)			(3,186)	(2,528)
Net income (loss)	(3,075)			(2,508)			(9,110)	(7,234)
Total assets	19,048			24,587			19,048	24,587
Goodwill	$ 0			$ 0			$ 0	$ 0