Fair Value Measurements - Estimated Fair Values and Related Carrying Amounts of Financial Instruments (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Held-to-maturity securities	$ 322,838	$ 373,298
Carrying Amount
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	513,804	598,042
Interest bearing balances due from banks - time	3,954	3,314
Held-to-maturity securities	323,306	368,058
Mortgage loans held for sale	48,195	24,038
Interest receivable	51,509	43,528
Federal funds purchased and securities sold under agreements to repurchase	109,213	122,444
Other borrowings	1,420,917	1,380,024
Subordinated notes and debentures	372,934	140,565
Interest payable	13,197	4,564
Carrying Amount | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,778,670	2,665,249
Carrying Amount | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	6,776,330	6,494,896
Carrying Amount | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,533,506	1,932,730
Carrying Amount | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	8,067,916	5,663,941
Carrying Amount | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	3,734,921	5,074,076
Fair Value Measurements
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Cash and cash equivalents	513,804	598,042
Interest bearing balances due from banks - time	3,954	3,314
Held-to-maturity securities	322,838	373,298
Mortgage loans held for sale	48,195	24,038
Interest receivable	51,509	43,528
Federal funds purchased and securities sold under agreements to repurchase	109,213	122,444
Other borrowings	1,417,166	1,381,365
Subordinated notes and debentures	310,701	136,474
Interest payable	13,197	4,564
Fair Value Measurements | Non-interest bearing transaction accounts
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,778,670	2,665,249
Fair Value Measurements | Interest bearing transaction accounts and savings deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	6,776,330	6,494,896
Fair Value Measurements | Time deposits
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Deposits	2,508,228	1,915,539
Fair Value Measurements | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	7,996,273	5,646,505
Fair Value Measurements | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	3,701,755	5,058,455
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	513,804	598,042
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	3,954	3,314
Held-to-maturity securities	322,838	373,298
Mortgage loans held for sale	0	0
Interest receivable	51,509	43,528
Federal funds purchased and securities sold under agreements to repurchase	109,213	122,444
Other borrowings	1,417,166	1,381,365
Subordinated notes and debentures	310,701	136,474
Interest payable	13,197	4,564
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	2,778,670	2,665,249
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	6,776,330	6,494,896
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	48,195	24,038
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	2,508,228	1,915,539
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Legacy loans, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	7,996,273	5,646,505
Fair Value Measurements | Significant Unobservable Inputs (Level 3) | Loans acquired, net
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Loans, net	$ 3,701,755	$ 5,058,455