Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 938,309,000	$ 962,731,000
Other Assets, Fair Value Disclosure	60,981,000	27,689,000
Other investments	12,227,000	16,761,000
Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	930,020,000	955,266,000
Other Assets, Fair Value Disclosure	54,362,000	21,894,000
Other investments	10,557,000	15,091,000
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	8,289,000	7,465,000
Other Assets, Fair Value Disclosure	$ 6,619,000	$ 5,795,000
Other Assets [Member] | Par Value [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Equity Securities, FV-NI, Measurement Input [Extensible List]	Par Value	Par Value
Equity Securities [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Other investments	$ 1,670,000	$ 1,670,000
Equity Securities [Member] | Par Value [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Equity Securities, FV-NI, Measurement Input [Extensible List]	Carrying Value	Carrying Value
Impaired Loans [Member] | Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure		$ 2,239,000
Equity Securities, FV-NI, Measurement Input [Extensible List]		Discounted Cash Flow
Impaired Loans [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 798,000	$ 884,000
Equity Securities, FV-NI, Measurement Input [Extensible List]	Gas Reserve study	Gas Reserve study
Impaired Loans [Member] | Discount Rates [Member] | Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Equity Securities, FV-NI, Measurement Input [Extensible List]		Discount Rate
Impaired Loans [Member] | Discount Rates [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Equity Securities, FV-NI, Measurement Input [Extensible List]	Discount rate	Discount rate
Impaired Loans [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Equity Securities, FV-NI, Measurement Input [Extensible List]	Gas per MMBTU	Gas per MMBTU
Impaired Loans [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Equity Securities, FV-NI, Measurement Input [Extensible List]	Oil per BBL/d	Oil per BBL/d
Fair Value, Nonrecurring [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 50,466,000	$ 32,393,000
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	36,862,000	14,875,000
Fair Value, Nonrecurring [Member] | Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 13,604,000	$ 17,518,000
Maximum [Member] | Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Input, Discount Rate	9.50%	9.50%
Maximum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 1.48	$ 3.49
Maximum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 36.00	$ 53.14
Weighted Average [Member] | Impaired Loans [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Input, Discount Rate	10.00%	10.00%
Minimum [Member] | Discounted Cash Flow [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Input, Discount Rate	3.84%	1.90%
Minimum [Member] | Gas Per Million Cubic Feet [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 1.46	$ 2.61
Minimum [Member] | Oil Per Barrels Per Day [Member] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 36.00	$ 47.09