Non-Covered Loans and Allowance for Non-Covered Loan Losses - Aging (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	$ 6,216,809	$ 6,273,669
Noncovered
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	32,850	16,563
Current Loans	6,156,068	6,219,902
Total loans	6,216,809	6,273,669
Accruing Loans Past Due 90 Days or More	791	640
Noncovered | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	17,148	6,513
Noncovered | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	9,510	1,723
Noncovered | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	6,192	8,327
Noncovered | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	27,891	37,204
Noncovered | Secured
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	14,738	8,086
Current Loans	1,508,386	1,544,131
Total loans	1,528,502	1,558,316
Accruing Loans Past Due 90 Days or More	1	640
Noncovered | Secured | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3,900	2,060
Noncovered | Secured | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	8,022	312
Noncovered | Secured | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,816	5,714
Noncovered | Secured | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	5,378	6,099
Noncovered | Unsecured
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	236	642
Current Loans	121,358	122,247
Total loans	121,594	122,889
Noncovered | Unsecured | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	230	642
Noncovered | Unsecured | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	6
Noncovered | Secured by Commercial Properties
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3,466	2,637
Current Loans	2,271,309	2,213,331
Total loans	2,291,891	2,235,643
Noncovered | Secured by Commercial Properties | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	1,236	442
Noncovered | Secured by Commercial Properties | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,230	2,195
Noncovered | Secured by Commercial Properties | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	17,116	19,675
Noncovered | Secured by Residential Properties
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	5,835	3,198
Current Loans	749,749	762,818
Total loans	759,712	775,881
Accruing Loans Past Due 90 Days or More	790
Noncovered | Secured by Residential Properties | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3,322	1,490
Noncovered | Secured by Residential Properties | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	1,367	1,290
Noncovered | Secured by Residential Properties | Financing Receivables, Equal to Greater than 90 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	1,146	418
Noncovered | Secured by Residential Properties | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	4,128	9,865
Noncovered | Residential Construction Loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	264	315
Current Loans	210,469	176,937
Total loans	210,733	177,252
Noncovered | Residential Construction Loans | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	264	315
Noncovered | Commercial construction loans and land development
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	8,098	1,471
Current Loans	709,614	782,444
Total loans	718,975	785,353
Noncovered | Commercial construction loans and land development | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	8,003	1,370
Noncovered | Commercial construction loans and land development | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	95	101
Noncovered | Commercial construction loans and land development | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	1,263	1,438
Noncovered | Consumer
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	213	214
Current Loans	38,679	40,105
Total loans	38,898	40,446
Noncovered | Consumer | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	193	194
Noncovered | Consumer | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	20	20
Noncovered | Consumer | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	6	127
Noncovered | Broker-dealer
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Current Loans	546,504	577,889
Total loans	546,504	577,889
Noncovered | PrimeLending | U.S. Government Agencies
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Accruing Loans Past Due 90 Days or More	76,800	84,500
Unpaid principal balance loans past due 90 days or more	$ 77,500	$ 85,200