Investment Securities, Securities Held-to-Maturity (Details) - USD ($)		3 Months Ended
		Mar. 31, 2021	Dec. 31, 2020	Mar. 31, 2020
Amortized cost, estimated fair values and unrealized gains and losses of investments classified as held-to-maturity [Abstract]
Amortized cost		$ 385,054,000	$ 68,933,000	$ 58,701,000
Gross unrealized gains		905,000	1,116,000	812,000
Gross unrealized losses		11,131,000	0	10,000
Fair value		374,828,000	70,049,000	59,503,000
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		70,454,000	68,933,000	58,701,000
Gross unrealized gains		905,000	1,116,000	812,000
Gross unrealized losses		0	0	10,000
Fair value		71,359,000	$ 70,049,000	$ 59,503,000
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]	314,600,000
Gross unrealized gains	[1],[2]	0
Gross unrealized losses	[1],[2]	11,131,000
Fair value	[1],[2]	$ 303,469,000

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