Non-Covered Loans and Allowance for Non-Covered Loan Losses - Internal Risk Grades (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Internal risk grades of non-covered loans
Total loans	$ 6,796,278	$ 6,273,669
Noncovered
Internal risk grades of non-covered loans
Total loans	6,796,278	6,273,669
Noncovered | Pass
Internal risk grades of non-covered loans
Total loans	6,621,868	6,092,219
Noncovered | Special Mention
Internal risk grades of non-covered loans
Total loans	7,411	27,260
Noncovered | Substandard
Internal risk grades of non-covered loans
Total loans	134,446	116,986
Noncovered | Secured
Internal risk grades of non-covered loans
Total loans	1,570,771	1,558,316
Noncovered | Secured | Pass
Internal risk grades of non-covered loans
Total loans	1,499,468	1,483,502
Noncovered | Secured | Special Mention
Internal risk grades of non-covered loans
Total loans	7,289	17,354
Noncovered | Secured | Substandard
Internal risk grades of non-covered loans
Total loans	58,486	51,361
Noncovered | Unsecured
Internal risk grades of non-covered loans
Total loans	151,326	122,889
Noncovered | Unsecured | Pass
Internal risk grades of non-covered loans
Total loans	148,499	121,774
Noncovered | Unsecured | Special Mention
Internal risk grades of non-covered loans
Total loans	122
Noncovered | Unsecured | Substandard
Internal risk grades of non-covered loans
Total loans	1,320	1,115
Noncovered | Secured by Commercial Properties
Internal risk grades of non-covered loans
Total loans	2,478,803	2,235,643
Noncovered | Secured by Commercial Properties | Pass
Internal risk grades of non-covered loans
Total loans	2,399,783	2,154,595
Noncovered | Secured by Commercial Properties | Special Mention
Internal risk grades of non-covered loans
Total loans		7,647
Noncovered | Secured by Commercial Properties | Substandard
Internal risk grades of non-covered loans
Total loans	57,950	53,726
Noncovered | Secured by Residential Properties
Internal risk grades of non-covered loans
Total loans	902,954	775,881
Noncovered | Secured by Residential Properties | Pass
Internal risk grades of non-covered loans
Total loans	885,974	756,091
Noncovered | Secured by Residential Properties | Substandard
Internal risk grades of non-covered loans
Total loans	13,127	9,925
Noncovered | Residential Construction Loans
Internal risk grades of non-covered loans
Total loans	241,640	177,252
Noncovered | Residential Construction Loans | Pass
Internal risk grades of non-covered loans
Total loans	241,640	177,252
Noncovered | Commercial construction loans and land development
Internal risk grades of non-covered loans
Total loans	810,769	785,353
Noncovered | Commercial construction loans and land development | Pass
Internal risk grades of non-covered loans
Total loans	806,594	780,905
Noncovered | Commercial construction loans and land development | Special Mention
Internal risk grades of non-covered loans
Total loans		2,259
Noncovered | Commercial construction loans and land development | Substandard
Internal risk grades of non-covered loans
Total loans	3,468	751
Noncovered | Consumer
Internal risk grades of non-covered loans
Total loans	46,739	40,446
Noncovered | Consumer | Pass
Internal risk grades of non-covered loans
Total loans	46,634	40,211
Noncovered | Consumer | Substandard
Internal risk grades of non-covered loans
Total loans	95	108
Noncovered | Broker-dealer
Internal risk grades of non-covered loans
Total loans	593,276	577,889
Noncovered | Broker-dealer | Pass
Internal risk grades of non-covered loans
Total loans	593,276	577,889
Noncovered | PCI loans
Internal risk grades of non-covered loans
Total loans	32,553	37,204
Noncovered | PCI loans | Secured
Internal risk grades of non-covered loans
Total loans	5,528	6,099
Noncovered | PCI loans | Unsecured
Internal risk grades of non-covered loans
Total loans	1,385
Noncovered | PCI loans | Secured by Commercial Properties
Internal risk grades of non-covered loans
Total loans	21,070	19,675
Noncovered | PCI loans | Secured by Residential Properties
Internal risk grades of non-covered loans
Total loans	3,853	9,865
Noncovered | PCI loans | Commercial construction loans and land development
Internal risk grades of non-covered loans
Total loans	707	1,438
Noncovered | PCI loans | Consumer
Internal risk grades of non-covered loans
Total loans	$ 10	$ 127