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Fair Value Measurements - Balances and Levels of Assets Measured at Fair Value on Recurring and Nonrecurring Basis (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Financial assets:
Mortgage servicing rights, net	$ 93,160	$ 76,107	$ 33,081
Impaired loans	9,018	9,373
Available-for-sale securities, at fair value		543,992
Fair Value, Measurements, Recurring
Financial assets:
Investment securities	597,347
Loans held for sale	414,518	526,185
Mortgage servicing rights, net	93,160	76,107
Derivatives assets	14,904	9,690
Available-for-sale securities, at fair value		543,992
Financial liabilities:
Derivatives liabilities	3,655	1,699
Fair Value, Measurements, Recurring \| U.S. Government Agency Securities
Financial assets:
Investment securities	982
Available-for-sale securities, at fair value		986
Fair Value, Measurements, Recurring \| Mortgage-backed Securities
Financial assets:
Investment securities	472,930
Available-for-sale securities, at fair value		418,781
Fair Value, Measurements, Recurring \| Municipals, Tax Exempt
Financial assets:
Investment securities	113,158
Available-for-sale securities, at fair value		109,251
Fair Value, Measurements, Recurring \| Treasury Securities
Financial assets:
Investment securities	7,178
Available-for-sale securities, at fair value		7,252
Fair Value, Measurements, Recurring \| Equity Securities
Financial assets:
Investment securities	3,099
Available-for-sale securities, at fair value		7,722
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2)
Financial assets:
Investment securities	597,347
Loans held for sale	414,518	526,185
Derivatives assets	14,904	9,690
Available-for-sale securities, at fair value		540,388
Financial liabilities:
Derivatives liabilities	3,655	1,699
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| U.S. Government Agency Securities
Financial assets:
Investment securities	982
Available-for-sale securities, at fair value		986
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Mortgage-backed Securities
Financial assets:
Investment securities	472,930
Available-for-sale securities, at fair value		418,781
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Municipals, Tax Exempt
Financial assets:
Investment securities	113,158
Available-for-sale securities, at fair value		109,251
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Treasury Securities
Financial assets:
Investment securities	7,178
Available-for-sale securities, at fair value		7,252
Fair Value, Measurements, Recurring \| Significant Other Observable Inputs (Level 2) \| Equity Securities
Financial assets:
Investment securities	3,099
Available-for-sale securities, at fair value		4,118
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3)
Financial assets:
Mortgage servicing rights, net	93,160	76,107
Available-for-sale securities, at fair value		3,604
Fair Value, Measurements, Recurring \| Significant Unobservable Inputs (Level 3) \| Equity Securities
Financial assets:
Available-for-sale securities, at fair value		3,604
Fair Value, Measurements, Nonrecurring
Financial assets:
Other real estate owned	800	13,174
Impaired loans	144	77,027
Fair Value, Measurements, Nonrecurring \| Commercial and Industrial
Financial assets:
Impaired loans		1,971
Fair Value, Measurements, Nonrecurring \| Consumer and Other
Financial assets:
Impaired loans		25,320
Fair Value, Measurements, Nonrecurring \| Construction Loans \| Commercial and Industrial
Financial assets:
Impaired loans		4,211
Fair Value, Measurements, Nonrecurring \| 1-to-4 Family Mortgage \| Residential Real Estate
Financial assets:
Impaired loans	144	21,902
Fair Value, Measurements, Nonrecurring \| Owner Occupied \| Commercial Real Estate
Financial assets:
Impaired loans		10,030
Fair Value, Measurements, Nonrecurring \| Non-Owner Occupied \| Commercial Real Estate
Financial assets:
Impaired loans		13,593
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3)
Financial assets:
Other real estate owned	800	13,174
Impaired loans	144	77,027
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3) \| Commercial and Industrial
Financial assets:
Impaired loans		1,971
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3) \| Consumer and Other
Financial assets:
Impaired loans		25,320
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3) \| Construction Loans \| Commercial and Industrial
Financial assets:
Impaired loans		4,211
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3) \| 1-to-4 Family Mortgage \| Residential Real Estate
Financial assets:
Impaired loans	$ 144	21,902
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3) \| Owner Occupied \| Commercial Real Estate
Financial assets:
Impaired loans		10,030
Fair Value, Measurements, Nonrecurring \| Significant Unobservable Inputs (Level 3) \| Non-Owner Occupied \| Commercial Real Estate
Financial assets:
Impaired loans		$ 13,593