Segment Reporting - Financial Information for Operating Segments (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Financial information for the Company's operating segments
Net interest income (loss)	$ 92,389	$ 79,603	$ 181,629	$ 153,618
Provision for loan losses	1,810	1,750	3,560	3,250
Noninterest income	35,581	34,265	69,534	66,286
Noninterest expense	79,026	74,841	156,970	144,150
Income before income taxes	47,134	37,277	90,633	72,504
Income tax expense (benefit)	10,424	11,993	20,097	23,248
Net income (loss)	36,710	25,284	70,536	49,256
Total assets	10,544,475	8,872,272	10,544,475	8,872,272	$ 9,829,981
Goodwill	611,046	470,534	611,046	470,534	611,046
Community Banks
Financial information for the Company's operating segments
Goodwill	608,279		608,279		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)	94,676	81,392	186,103	157,348
Provision for loan losses	1,810	1,750	3,560	3,250
Noninterest income	29,949	28,592	57,867	55,170
Noninterest expense	73,628	70,018	146,261	134,239
Income before income taxes	49,187	38,216	94,149	75,029
Income tax expense (benefit)	11,165	12,712	21,332	24,822
Net income (loss)	38,022	25,504	72,817	50,207
Total assets	10,439,785	8,776,737	10,439,785	8,776,737
Goodwill	608,279	467,767	608,279	467,767
Operating Segments | Insurance
Financial information for the Company's operating segments
Net interest income (loss)	118	124	224	216
Provision for loan losses	0	0	0	0
Noninterest income	2,148	2,264	4,920	4,813
Noninterest expense	1,819	1,766	3,550	3,458
Income before income taxes	447	622	1,594	1,571
Income tax expense (benefit)	116	238	413	613
Net income (loss)	331	384	1,181	958
Total assets	24,513	24,746	24,513	24,746
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)	315	524	628	1,011
Provision for loan losses	0	0		0
Noninterest income	3,714	3,267	7,241	6,386
Noninterest expense	3,213	2,905	6,605	5,901
Income before income taxes	816	886	1,264	1,496
Income tax expense (benefit)	0	0	0	0
Net income (loss)	816	886	1,264	1,496
Total assets	61,869	58,156	61,869	58,156
Goodwill	0	0	0	0
Other
Financial information for the Company's operating segments
Net interest income (loss)	(2,720)	(2,437)	(5,326)	(4,957)
Provision for loan losses	0	0	0	0
Noninterest income	(230)	142	(494)	(83)
Noninterest expense	366	152	554	552
Income before income taxes	(3,316)	(2,447)	(6,374)	(5,592)
Income tax expense (benefit)	(857)	(957)	(1,648)	(2,187)
Net income (loss)	(2,459)	(1,490)	(4,726)	(3,405)
Total assets	18,308	12,633	18,308	12,633
Goodwill	$ 0	$ 0	$ 0	$ 0