Fair Value Measurements - Fair Value Inputs, Quantitative Information (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Mar. 31, 2019	Dec. 31, 2017
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Write downs on OREO properties	$ 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		43.00%
Discount Rate for OREO Level 3 Fair Value		96.00%
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		10.00%
Discount Rate for OREO Level 3 Fair Value		39.00%
Discount Rate for Level 3 Fair Value Disclosure		13.00%
Carrying Amount
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Net loans held for investment		$ 8,397,400,000	$ 8,386,800,000
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		$ 500,000
Estimated Fair Value
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Net loans held for investment		8,439,700,000	8,408,600,000
Estimated Fair Value | 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		5,500,000		4,900,000
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)
Partial loan charge-offs		6,200,000		5,400,000
Long-lived assets to be disposed, fair value disclosure		4,900,000	5,500,000
Impaired loans		24,100,000
Other real estate owned		600,000	2,000,000
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		44,200,000		36,300,000
Fair Value of long-lived assets to be disposed of by sale		6,000,000		5,400,000
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		24,100,000	30,900,000
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)
Long-lived assets to be disposed, fair value disclosure		4,900,000	5,500,000
Impaired loans		$ 30,900,000	30,900,000	$ 24,100,000
Other real estate owned			$ 2,000,000
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		26.00%