Fair Values of Assets and Liabilities - Quantitative Inputs and Assumptions Used in Level 3 Fair Value Measurements (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 1,114,587,000	$ 938,309,000
Other Assets, Fair Value Disclosure	41,786,000	60,981,000
Other investments	12,838,000	12,227,000
Fair Value, Inputs, Level 2 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	1,098,436,000	930,020,000
Other Assets, Fair Value Disclosure	26,805,000	54,362,000
Other investments	11,668,000	10,557,000
Fair Value, Inputs, Level 3 [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	16,151,000	8,289,000
Other Assets, Fair Value Disclosure	$ 14,981,000	$ 6,619,000
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	$ 1,170,000	$ 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] | Reserve [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Assets, Fair Value Disclosure	$ 598,000	$ 798,000
Equity Securities, FV-NI, Measurement Input [Extensible List]	Gas Reserve Study	Gas Reserve study
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	$ 55,514,000	$ 50,466,000
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	43,267,000	36,862,000
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	12,247,000	13,604,000
Minimum [Member] | Impaired Loans [Member] | Reserve [Member] | Gas Per Million Cubic Feet [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	2.00	1.46
Minimum [Member] | Impaired Loans [Member] | Reserve [Member] | Oil Per Barrels Per Day [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 50.00	$ 36.00
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%
Maximum [Member] | Impaired Loans [Member] | Reserve [Member] | Gas Per Million Cubic Feet [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 2.00	$ 1.48
Maximum [Member] | Impaired Loans [Member] | Reserve [Member] | Oil Per Barrels Per Day [Member]
Fair Value Inputs, Assets, Quantitative Information [Line Items] (Deprecated 2018-01-31)
Fair Value Weighted Average Inputs Price	$ 50.00	$ 36.00