Non-Covered Loans and Allowance for Non-Covered Loan Losses - Internal Risk Grades (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Internal risk grades of non-covered loans
Total loans	$ 6,216,809	$ 6,273,669
Noncovered
Internal risk grades of non-covered loans
Total loans	6,216,809	6,273,669
Noncovered | Pass
Internal risk grades of non-covered loans
Total loans	6,031,250	6,092,219
Noncovered | Special Mention
Internal risk grades of non-covered loans
Total loans	11,372	27,260
Noncovered | Substandard
Internal risk grades of non-covered loans
Total loans	146,296	116,986
Noncovered | Secured
Internal risk grades of non-covered loans
Total loans	1,528,502	1,558,316
Noncovered | Secured | Pass
Internal risk grades of non-covered loans
Total loans	1,448,566	1,483,502
Noncovered | Secured | Special Mention
Internal risk grades of non-covered loans
Total loans	1,090	17,354
Noncovered | Secured | Substandard
Internal risk grades of non-covered loans
Total loans	73,468	51,361
Noncovered | Unsecured
Internal risk grades of non-covered loans
Total loans	121,594	122,889
Noncovered | Unsecured | Pass
Internal risk grades of non-covered loans
Total loans	120,779	121,774
Noncovered | Unsecured | Substandard
Internal risk grades of non-covered loans
Total loans	815	1,115
Noncovered | Secured by Commercial Properties
Internal risk grades of non-covered loans
Total loans	2,291,891	2,235,643
Noncovered | Secured by Commercial Properties | Pass
Internal risk grades of non-covered loans
Total loans	2,206,232	2,154,595
Noncovered | Secured by Commercial Properties | Special Mention
Internal risk grades of non-covered loans
Total loans	7,314	7,647
Noncovered | Secured by Commercial Properties | Substandard
Internal risk grades of non-covered loans
Total loans	61,229	53,726
Noncovered | Secured by Residential Properties
Internal risk grades of non-covered loans
Total loans	759,712	775,881
Noncovered | Secured by Residential Properties | Pass
Internal risk grades of non-covered loans
Total loans	742,708	756,091
Noncovered | Secured by Residential Properties | Special Mention
Internal risk grades of non-covered loans
Total loans	2,968
Noncovered | Secured by Residential Properties | Substandard
Internal risk grades of non-covered loans
Total loans	9,908	9,925
Noncovered | Residential Construction Loans
Internal risk grades of non-covered loans
Total loans	210,733	177,252
Noncovered | Residential Construction Loans | Pass
Internal risk grades of non-covered loans
Total loans	210,733	177,252
Noncovered | Commercial construction loans and land development
Internal risk grades of non-covered loans
Total loans	718,975	785,353
Noncovered | Commercial construction loans and land development | Pass
Internal risk grades of non-covered loans
Total loans	716,985	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	727	751
Noncovered | Consumer
Internal risk grades of non-covered loans
Total loans	38,898	40,446
Noncovered | Consumer | Pass
Internal risk grades of non-covered loans
Total loans	38,743	40,211
Noncovered | Consumer | Substandard
Internal risk grades of non-covered loans
Total loans	149	108
Noncovered | Broker-dealer
Internal risk grades of non-covered loans
Total loans	546,504	577,889
Noncovered | Broker-dealer | Pass
Internal risk grades of non-covered loans
Total loans	546,504	577,889
Noncovered | PCI loans
Internal risk grades of non-covered loans
Total loans	27,891	37,204
Noncovered | PCI loans | Secured
Internal risk grades of non-covered loans
Total loans	5,378	6,099
Noncovered | PCI loans | Secured by Commercial Properties
Internal risk grades of non-covered loans
Total loans	17,116	19,675
Noncovered | PCI loans | Secured by Residential Properties
Internal risk grades of non-covered loans
Total loans	4,128	9,865
Noncovered | PCI loans | Commercial construction loans and land development
Internal risk grades of non-covered loans
Total loans	1,263	1,438
Noncovered | PCI loans | Consumer
Internal risk grades of non-covered loans
Total loans	$ 6	$ 127