Disclosure About Fair Values of Financial Instruments - Summary of Significant Unobservable Inputs Used in Level 3 Fair Value Measurements for Financial Assets Measured at Fair Value On a Nonrecurring Basis (Details) - USD ($) $ in Thousands		12 Months Ended
		Dec. 31, 2018		Dec. 31, 2017
Discounted Cash Flow | Net Recoverable Oil and Gas Reserves and Forward Looking Commodity Prices
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value inputs, discount rate		10.00%
Level 3
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs		Estimated closing costs
Range		10.00%
Level 3 | Impaired Loans Net Of Specific Allowance
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value		$ 71,741		$ 65,088
Valuation Methods		Appraised value, as adjusted		Appraised value, as adjusted
Unobservable Inputs		Discount to fair value		Discount to fair value
Level 3 | Impaired Loans Net Of Specific Allowance | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		20.00%		50.00%
Level 3 | Impaired Loans Net Of Specific Allowance | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		0.00%		0.00%
Level 3 | Discounted Cash Flow
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Valuation Methods		Discounted cash flow		Discounted cash flow
Unobservable Inputs		Discount rates - 2.9% to 8.7%		Discount rates - 3.6% to 8.0%
Level 3 | Discounted Cash Flow | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		20.00%	[1]	1.00%	[2]
Fair value inputs, discount rate		8.70%		8.00%
Level 3 | Discounted Cash Flow | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		10.00%	[1]	0.00%	[2]
Fair value inputs, discount rate		2.90%		3.80%
Level 3 | Discounted Cash Flow | Net Recoverable Oil and Gas Reserves and Forward Looking Commodity Prices
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Valuation Methods		Discounted cash flow		Discounted cash flow
Unobservable Inputs		Net recoverable oil and gas reserves and forward-looking commodity prices. Discount rate - 10%		Net recoverable oil and gas reserves and forward-looking commodity prices. Discount rate - 9%
Fair value inputs, discount rate		10.00%		9.00%
Level 3 | Discounted Cash Flow | Net Recoverable Oil and Gas Reserves and Forward Looking Commodity Prices | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		10.00%		29.00%	[2]
Level 3 | Discounted Cash Flow | Net Recoverable Oil and Gas Reserves and Forward Looking Commodity Prices | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		0.00%		0.00%	[2]
Level 3 | Enterprise Value
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Valuation Methods		Enterprise value
Unobservable Inputs		Exit multiples
Level 3 | Enterprise Value | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurements, percentage of enterprise value	[1]	15.00%
Level 3 | Enterprise Value | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Fair value measurements, percentage of enterprise value	[1]	0.00%
Level 3 | Discount of Fair Value | Other
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Carrying Value		$ 2,406		$ 7,605
Valuation Methods		Appraised value, as adjusted		Appraised value, as adjusted
Unobservable Inputs		Discount to fair value		Discount of fair value
Level 3 | Discount of Fair Value | Other | Maximum
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		20.00%		20.00%
Level 3 | Discount of Fair Value | Other | Minimum [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Range		0.00%		0.00%
Level 3 | Estimated Closing Costs
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Unobservable Inputs		Estimated closing costs		Estimated closing costs
Range		10.00%		10.00%

[1] Represents difference of unpaid balance to fair value.

[2] Represents fair value as a percent of the unpaid principal balance.