Loans - Commercial Loans by Risk Rating Categories (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 13,833,368	$ 12,990,779
Originated
Financing Receivable, Recorded Investment [Line Items]
Total loans	9,156,096	7,458,401
Originated | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,255,444	1,901,526
Originated | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,472,224	1,921,799
Originated | Real estate construction and land development
Financing Receivable, Recorded Investment [Line Items]
Total loans	421,764	281,724
Acquired
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,677,272	5,532,378
Commercial loan portfolio
Financing Receivable, Recorded Investment [Line Items]
Total loans	8,137,356	7,594,212
Commercial loan portfolio | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	7,756,813	7,167,397
Commercial loan portfolio | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	159,509	192,013
Commercial loan portfolio | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	218,857	233,479
Commercial loan portfolio | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,177	1,323
Commercial loan portfolio | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans	3,319,965	3,217,300
Commercial loan portfolio | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,315,978	3,973,140
Commercial loan portfolio | Real estate construction and land development
Financing Receivable, Recorded Investment [Line Items]
Total loans	501,413	403,772
Commercial loan portfolio | Originated
Financing Receivable, Recorded Investment [Line Items]
Total loans	5,149,432	4,105,049
Commercial loan portfolio | Originated | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	4,997,274	3,934,033
Commercial loan portfolio | Originated | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	54,945	81,947
Commercial loan portfolio | Originated | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	95,201	87,999
Commercial loan portfolio | Originated | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,012	1,070
Commercial loan portfolio | Originated | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,255,444	1,901,526
Commercial loan portfolio | Originated | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,170,377	1,803,750
Commercial loan portfolio | Originated | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	34,571	44,809
Commercial loan portfolio | Originated | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	49,504	51,898
Commercial loan portfolio | Originated | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	992	1,069
Commercial loan portfolio | Originated | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,472,224	1,921,799
Commercial loan portfolio | Originated | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,405,684	1,849,315
Commercial loan portfolio | Originated | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	20,037	36,981
Commercial loan portfolio | Originated | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	45,483	35,502
Commercial loan portfolio | Originated | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,020	1
Commercial loan portfolio | Originated | Real estate construction and land development
Financing Receivable, Recorded Investment [Line Items]
Total loans	421,764	281,724
Commercial loan portfolio | Originated | Real estate construction and land development | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	421,213	280,968
Commercial loan portfolio | Originated | Real estate construction and land development | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	337	157
Commercial loan portfolio | Originated | Real estate construction and land development | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	214	599
Commercial loan portfolio | Originated | Real estate construction and land development | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial loan portfolio | Acquired
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,987,924	3,489,163
Commercial loan portfolio | Acquired | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,759,539	3,233,364
Commercial loan portfolio | Acquired | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	104,564	110,066
Commercial loan portfolio | Acquired | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	123,656	145,480
Commercial loan portfolio | Acquired | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	165	253
Commercial loan portfolio | Acquired | Commercial
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,064,521	1,315,774
Commercial loan portfolio | Acquired | Commercial | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	990,101	1,218,848
Commercial loan portfolio | Acquired | Commercial | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	34,169	46,643
Commercial loan portfolio | Acquired | Commercial | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	40,251	50,283
Commercial loan portfolio | Acquired | Commercial | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	0	0
Commercial loan portfolio | Acquired | Commercial real estate
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,843,754	2,051,341
Commercial loan portfolio | Acquired | Commercial real estate | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,693,878	1,897,011
Commercial loan portfolio | Acquired | Commercial real estate | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	68,261	61,441
Commercial loan portfolio | Acquired | Commercial real estate | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	81,450	92,636
Commercial loan portfolio | Acquired | Commercial real estate | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	165	253
Commercial loan portfolio | Acquired | Real estate construction and land development
Financing Receivable, Recorded Investment [Line Items]
Total loans	79,649	122,048
Commercial loan portfolio | Acquired | Real estate construction and land development | Pass
Financing Receivable, Recorded Investment [Line Items]
Total loans	75,560	117,505
Commercial loan portfolio | Acquired | Real estate construction and land development | Special Mention
Financing Receivable, Recorded Investment [Line Items]
Total loans	2,134	1,982
Commercial loan portfolio | Acquired | Real estate construction and land development | Substandard
Financing Receivable, Recorded Investment [Line Items]
Total loans	1,955	2,561
Commercial loan portfolio | Acquired | Real estate construction and land development | Doubtful
Financing Receivable, Recorded Investment [Line Items]
Total loans	$ 0	$ 0