Segment Reporting - Financial Information for Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Financial information for the Company's operating segments
Net interest income (loss)	$ 99,439	$ 90,017	$ 281,068	$ 243,635
Provision for loan losses	2,250	2,150	5,810	5,400
Noninterest income	38,053	33,413	107,587	99,699
Noninterest expense	94,746	80,660	251,716	224,810
Income before income taxes	40,496	40,620	131,129	113,124
Income tax expense (benefit)	8,532	14,199	28,629	37,447
Net income (loss)	31,964	26,421	102,500	75,677
Total assets	12,746,939	10,323,687	12,746,939	10,323,687	$ 9,829,981
Goodwill	927,261	611,046	927,261	611,046	611,046
Community Banks
Financial information for the Company's operating segments
Goodwill	924,494		924,494		608,279
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)	101,970	92,007	288,073	249,355
Provision for loan losses	2,250	2,150	5,810	5,400
Noninterest income	32,140	28,120	90,007	83,290
Noninterest expense	89,370	75,681	235,631	209,920
Income before income taxes	42,490	42,296	136,639	117,325
Income tax expense (benefit)	9,226	15,199	30,558	40,021
Net income (loss)	33,264	27,097	106,081	77,304
Total assets	12,634,614	10,216,826	12,634,614	10,216,826
Goodwill	924,494	608,279	924,494	608,279
Operating Segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	124	114	348	330
Provision for loan losses	0	0	0	0
Noninterest income	2,488	2,394	7,408	7,207
Noninterest expense	1,899	1,805	5,449	5,263
Income before income taxes	713	703	2,307	2,274
Income tax expense (benefit)	186	275	599	888
Net income (loss)	527	428	1,708	1,386
Total assets	25,236	25,729	25,236	25,729
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)	324	564	952	1,575
Provision for loan losses	0	0	0	0
Noninterest income	3,641	3,213	10,882	9,599
Noninterest expense	3,284	2,887	9,889	8,788
Income before income taxes	681	890	1,945	2,386
Income tax expense (benefit)	0	0	0	0
Net income (loss)	681	890	1,945	2,386
Total assets	62,502	59,703	62,502	59,703
Goodwill	0	0	0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(2,979)	(2,668)	(8,305)	(7,625)
Provision for loan losses	0	0	0	0
Noninterest income	(216)	(314)	(710)	(397)
Noninterest expense	193	287	747	839
Income before income taxes	(3,388)	(3,269)	(9,762)	(8,861)
Income tax expense (benefit)	(880)	(1,275)	(2,528)	(3,462)
Net income (loss)	(2,508)	(1,994)	(7,234)	(5,399)
Total assets	24,587	21,429	24,587	21,429
Goodwill	$ 0	$ 0	$ 0	$ 0