Segment Reporting (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financial information for operating segments
Net interest income	$ 93,262	$ 90,017	$ 79,603	$ 74,015	$ 78,049	$ 75,731	$ 77,157	$ 70,054	$ 336,897	$ 300,991	$ 241,358
Provision for loan losses	2,150	2,150	1,750	1,500	1,650	2,650	1,430	1,800	7,550	7,530	4,750
Noninterest income	32,441	33,413	34,265	32,021	30,255	38,272	35,586	33,302	132,140	137,415	108,270
Noninterest expense	76,808	80,660	74,841	69,309	71,558	76,468	77,259	69,814	301,618	295,099	245,114
Income before income taxes	46,745	40,620	37,277	35,227	35,096	34,885	34,054	31,742	159,869	135,777	99,764
Income taxes	30,234	14,199	11,993	11,255	11,461	11,706	11,154	10,526	67,681	44,847	31,750
Net income	16,511	$ 26,421	$ 25,284	$ 23,972	23,635	$ 23,179	$ 22,900	$ 21,216	92,188	90,930	68,014
Total assets	9,829,981				8,699,851				9,829,981	8,699,851	7,926,496
Goodwill	611,046				470,534				611,046	470,534	445,871
Community Banks
Financial information for operating segments
Goodwill	608,279				467,767				608,279	467,767	443,104
Insurance
Financial information for operating segments
Goodwill	2,767				2,767				2,767	2,767	2,767
Operating Segments | Community Banks
Financial information for operating segments
Net interest income									344,499	305,583	244,242
Provision for loan losses									7,550	7,530	4,752
Noninterest income									110,308	114,615	88,498
Noninterest expense									281,698	276,260	228,248
Income before income taxes									165,559	136,408	99,740
Income taxes									70,257	46,352	32,941
Net income									95,302	90,056	66,799
Total assets	9,717,779				8,602,022				9,717,779	8,602,022	7,833,084
Goodwill	608,279				467,767				608,279	467,767	443,104
Operating Segments | Insurance
Financial information for operating segments
Net interest income									457	350	311
Provision for loan losses									0	0	0
Noninterest income									9,530	10,074	9,340
Noninterest expense									6,957	6,873	6,900
Income before income taxes									3,030	3,551	2,751
Income taxes									1,184	1,385	1,078
Net income									1,846	2,166	1,673
Total assets	26,470				23,693				26,470	23,693	22,550
Goodwill	2,767				2,767				2,767	2,767	2,767
Operating Segments | Wealth Management
Financial information for operating segments
Net interest income									2,160	1,846	1,688
Provision for loan losses									0	0	(2)
Noninterest income									12,863	12,354	10,559
Noninterest expense									11,785	11,099	9,130
Income before income taxes									3,238	3,101	3,119
Income taxes									0	0	0
Net income									3,238	3,101	3,119
Total assets	61,330				54,857				61,330	54,857	46,035
Goodwill	0				0				0	0	0
Other
Financial information for operating segments
Net interest income									(10,219)	(6,788)	(4,883)
Provision for loan losses									0	0	0
Noninterest income									(561)	372	(127)
Noninterest expense									1,178	867	836
Income before income taxes									(11,958)	(7,283)	(5,846)
Income taxes									(3,760)	(2,890)	(2,269)
Net income									(8,198)	(4,393)	(3,577)
Total assets	24,402				19,279				24,402	19,279	24,827
Goodwill	$ 0				$ 0				$ 0	$ 0	$ 0