Fair Value Measurements - Estimated Fair Values and Related Carrying Amounts of Financial Instruments (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 61,956	$ 290,830
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	491,161	833,458
Interest bearing balances due from banks - time	4,684	4,934
Held-to-maturity securities	61,435	289,194
Mortgage loans held for sale	18,480	26,799
Interest receivable	51,796	49,938
Federal funds purchased and securities sold under agreements to repurchase	120,213	95,792
Other borrowings	1,169,989	1,345,450
Subordinated notes and debentures	354,041	353,950
Interest payable	13,941	9,897
Carrying Amount | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,674,034	2,672,405
Carrying Amount | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	6,666,823	6,830,191
Carrying Amount | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,648,674	2,896,156
Carrying Amount | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	8,625,307	8,373,789
Carrying Amount | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	3,056,187	3,292,783
Fair Value Measurements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	491,161	833,458
Interest bearing balances due from banks - time	4,684	4,934
Held-to-maturity securities	61,956	290,830
Mortgage loans held for sale	18,480	26,799
Interest receivable	51,796	49,938
Federal funds purchased and securities sold under agreements to repurchase	120,213	95,792
Other borrowings	1,168,914	1,342,868
Subordinated notes and debentures	359,090	355,812
Interest payable	13,941	9,897
Fair Value Measurements | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,674,034	2,672,405
Fair Value Measurements | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	6,666,823	6,830,191
Fair Value Measurements | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,632,615	2,872,342
Fair Value Measurements | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	8,539,036	8,280,690
Fair Value Measurements | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	3,025,619	3,256,174
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	491,161	833,458
Interest bearing balances due from banks - time	0	0
Held-to-maturity securities	0	0
Mortgage loans held for sale	0	0
Interest receivable	0	0
Federal funds purchased and securities sold under agreements to repurchase	0	0
Other borrowings	0	0
Subordinated notes and debentures	0	0
Interest payable	0	0
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1) | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1) | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1) | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1) | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Fair Value Measurements | Quoted Prices in Active Markets for Identical Assets (Level 1) | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Fair Value Measurements | Significant Other Observable Inputs (Level 2)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Interest bearing balances due from banks - time	4,684	4,934
Held-to-maturity securities	61,956	290,830
Mortgage loans held for sale	0	0
Interest receivable	51,796	49,938
Federal funds purchased and securities sold under agreements to repurchase	120,213	95,792
Other borrowings	1,168,914	1,342,868
Subordinated notes and debentures	359,090	355,812
Interest payable	13,941	9,897
Fair Value Measurements | Significant Other Observable Inputs (Level 2) | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,674,034	2,672,405
Fair Value Measurements | Significant Other Observable Inputs (Level 2) | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	6,666,823	6,830,191
Fair Value Measurements | Significant Other Observable Inputs (Level 2) | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Fair Value Measurements | Significant Other Observable Inputs (Level 2) | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Fair Value Measurements | Significant Other Observable Inputs (Level 2) | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	0	0
Fair Value Measurements | Significant Unobservable Inputs (Level 3)
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	0	0
Interest bearing balances due from banks - time	0	0
Held-to-maturity securities	0	0
Mortgage loans held for sale	18,480	26,799
Interest receivable	0	0
Federal funds purchased and securities sold under agreements to repurchase	0	0
Other borrowings	0	0
Subordinated notes and debentures	0	0
Interest payable	0	0
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	0	0
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,632,615	2,872,342
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	8,539,036	8,280,690
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 3,025,619	$ 3,256,174