Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - USD ($)	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Write downs on OREO properties	$ 0	$ 0
Impaired Loans | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	0.00%
Impaired Loans | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	78.10%
Impaired Loans | Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	26.10%
Other Real Estate Owned | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	8.00%
Other Real Estate Owned | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	96.20%
Other Real Estate Owned | Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	11.60%
Asses to be Disposed of by Sale | Minimum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	0.00%
Asses to be Disposed of by Sale | Maximum | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	0.00%
Asses to be Disposed of by Sale | Weighted Average | Market Approach Valuation Technique
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Discount rate of long-lived assets to be disposed of by sale	0.00%
Carrying Amount
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net loans held for investment	$ 7,614,500,000		$ 7,542,200,000
Change During Period | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Fair Value of long-lived assets to be disposed of by sale			0
Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net loans held for investment	7,388,200,000		7,298,800,000
Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	36,000,000		32,600,000
Other real estate owned	1,300,000		1,300,000
Fair Value of long-lived assets to be disposed of by sale	800,000		800,000
Significant Unobservable Inputs (Level 3) | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans			32,600,000
Other real estate owned	1,300,000		1,300,000
Fair Value of long-lived assets to be disposed of by sale	800,000		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]
Impaired loans	57,700,000		54,800,000
Fair Value of long-lived assets to be disposed of by sale			800,000
Significant Unobservable Inputs (Level 3) | Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Net loans held for investment			32,600,000
Impaired loans	36,000,000
Significant Unobservable Inputs (Level 3) | Estimated Fair Value | Fair Value Measured on a Non-recurring Basis
Fair Value Inputs, Assets, Quantitative Information [Line Items]
Impaired loans	$ 36,000,000		$ 32,600,000