Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - USD ($)	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Write downs on OREO properties	$ 0	$ 0
Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	0.00%
Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Assets HFS Level 3 Fair Value	0.00%
Discount Rate for OREO Level 3 Fair Value	96.20%
Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Assets HFS Level 3 Fair Value	0.00%
Discount Rate for OREO Level 3 Fair Value	11.60%
Discount Rate for Level 3 Fair Value Disclosure	26.10%
Carrying Amount
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Net loans held for investment	$ 8,480,300,000		$ 7,542,200,000
Change During Period | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value of long-lived assets to be disposed of by sale	500,000		0
Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Net loans held for investment	8,406,800,000		7,298,800,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans	28,700,000		32,600,000
Other real estate owned	600,000		1,300,000
Fair Value of long-lived assets to be disposed of by sale	3,500,000		800,000
Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans			32,600,000
Other real estate owned	600,000		1,300,000
Fair Value of long-lived assets to be disposed of by sale	3,500,000		800,000
Significant Unobservable Inputs (Level 3) | Carrying Amount | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans	43,500,000		54,800,000
Fair Value of long-lived assets to be disposed of by sale	4,000,000		800,000
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Net loans held for investment			32,600,000
Impaired loans	28,700,000
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Impaired loans	$ 28,700,000		$ 32,600,000
Measurement Input, Discount Rate [Member] | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Assets HFS Level 3 Fair Value	0.00%
Discount Rate for OREO Level 3 Fair Value	8.00%
Measurement Input, Discount Rate [Member] | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Discount Rate for Level 3 Fair Value Disclosure	78.10%