Investment Securities, Securities Held-to-Maturity (Details) - USD ($)		3 Months Ended
		Mar. 31, 2021	Sep. 30, 2021	Dec. 31, 2020	Sep. 30, 2020
Amortized cost, estimated fair values and unrealized gains and losses of investments classified as held-to-maturity [Abstract]
Amortized cost			$ 550,618,000	$ 68,933,000	$ 69,913,000
Gross unrealized gains			816,000	1,116,000	1,142,000
Gross unrealized losses			11,126,000	0	0
Fair value			540,308,000	70,049,000	71,055,000
Obligations of States and Political Subdivisions [Member]
Amortized cost, estimated fair values and unrealized gains and losses of investments classified as held-to-maturity [Abstract]
Amortized cost			62,936,000	68,933,000	69,913,000
Gross unrealized gains			768,000	1,116,000	1,142,000
Gross unrealized losses			0	0	0
Fair value			63,704,000	$ 70,049,000	$ 71,055,000
Mortgage Backed Securities [Member]
Securities Transferred [Abstract]
Fair value of securities transferred to held-to-maturity category		$ 316,900,000
Unrealized pre-tax loss from securities transferred		$ 2,000
Amortized cost, estimated fair values and unrealized gains and losses of investments classified as held-to-maturity [Abstract]
Amortized cost	[1],[2]		487,682,000
Gross unrealized gains	[1],[2]		48,000
Gross unrealized losses	[1],[2]		11,126,000
Fair value	[1],[2]		$ 476,604,000

[1] All Mortgage-Backed securities were issued by an agency or government-sponsored entity of the U.S. government.

[2] During Q1 2021, the Company transferred $316.9 million of AFS securities to HTM.