Non-Covered Loans and Allowance for Non-Covered Loan Losses - Aging (Details) - USD ($) $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	$ 6,796,278	$ 6,273,669
Noncovered
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	60,543	16,563
Current Loans	6,703,182	6,219,902
Total loans	6,796,278	6,273,669
Accruing Loans Past Due 90 Days or More	4,232	640
Noncovered | Financing Receivables, 30 to 59 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	32,700	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	19,291	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	8,552	8,327
Noncovered | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	32,553	37,204
Noncovered | Secured
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	36,598	8,086
Current Loans	1,528,645	1,544,131
Total loans	1,570,771	1,558,316
Accruing Loans Past Due 90 Days or More	3,544	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	17,853	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	12,710	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	6,035	5,714
Noncovered | Secured | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	5,528	6,099
Noncovered | Unsecured
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,877	642
Current Loans	147,064	122,247
Total loans	151,326	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	2,417	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	460
Noncovered | Unsecured | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	1,385
Noncovered | Secured by Commercial Properties
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	11,921	2,637
Current Loans	2,445,812	2,213,331
Total loans	2,478,803	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	6,970	442
Noncovered | Secured by Commercial Properties | Financing Receivables, 60 to 89 Days Past Due
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	3,130
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	1,821	2,195
Noncovered | Secured by Commercial Properties | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	21,070	19,675
Noncovered | Secured by Residential Properties
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	2,607	3,198
Current Loans	896,494	762,818
Total loans	902,954	775,881
Accruing Loans Past Due 90 Days or More	688
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	1,248	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	663	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	696	418
Noncovered | Secured by Residential Properties | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	3,853	9,865
Noncovered | Residential Construction Loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	739	315
Current Loans	240,901	176,937
Total loans	241,640	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	739	315
Noncovered | Commercial construction loans and land development
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	4,135	1,471
Current Loans	805,927	782,444
Total loans	810,769	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	3,100	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	1,035	101
Noncovered | Commercial construction loans and land development | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	707	1,438
Noncovered | Consumer
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total Past Due Loans	1,666	214
Current Loans	45,063	40,105
Total loans	46,739	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	373	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	1,293	20
Noncovered | Consumer | PCI loans
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Total loans	10	127
Noncovered | Broker-dealer
Non-Covered Loans and Allowance for Non-Covered Loan Losses
Current Loans	593,276	577,889
Total loans	593,276	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	76,400	84,500
Unpaid principal balance loans past due 90 days or more	$ 77,200	$ 85,200