Investment Securities, Securities Held-to-Maturity (Details) - USD ($)		3 Months Ended
		Mar. 31, 2021	Jun. 30, 2021	Dec. 31, 2020	Jun. 30, 2020
Amortized cost, estimated fair values and unrealized gains and losses of investments classified as held-to-maturity [Abstract]
Amortized cost			$ 496,470,000	$ 68,933,000	$ 69,036,000
Gross unrealized gains			902,000	1,116,000	1,158,000
Gross unrealized losses			7,187,000	0	0
Fair value			490,185,000	70,049,000	70,194,000
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			68,471,000	68,933,000	69,036,000
Gross unrealized gains			895,000	1,116,000	1,158,000
Gross unrealized losses			0	0	0
Fair value			69,366,000	$ 70,049,000	$ 70,194,000
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]		427,999,000
Gross unrealized gains	[1],[2]		7,000
Gross unrealized losses	[1],[2]		7,187,000
Fair value	[1],[2]		$ 420,819,000

[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.