Investments (Aggregate Carrying And Approximate Market Values Of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Mar. 31, 2021	Dec. 31, 2020
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	$ 1,157,127	$ 1,130,155
Gross Unrealized Gains	32,972	49,336
Gross Unrealized Losses	4,854	702
Estimated Fair Value	1,185,245	1,178,789
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	273,855	266,483
Gross Unrealized Gains	7,082	11,467
Gross Unrealized Losses	1,184	139
Estimated Fair Value	279,753	277,811
Mortgage-backed securities: U.S. government agencies
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	834,608	815,682
Gross Unrealized Gains	24,328	34,807
Gross Unrealized Losses	3,382	105
Estimated Fair Value	855,554	850,384
Mortgage-backed securities: Private label
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	9,691	9,976
Gross Unrealized Gains	705	916
Gross Unrealized Losses	0	0
Estimated Fair Value	10,396	10,892
Trust preferred securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	4,560	4,557
Gross Unrealized Gains	0	0
Gross Unrealized Losses	285	457
Estimated Fair Value	4,275	4,100
Corporate securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	32,421	31,465
Gross Unrealized Gains	857	2,146
Gross Unrealized Losses	3	1
Estimated Fair Value	33,275	33,610
Total Debt Securities
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,155,135	1,128,163
Gross Unrealized Gains	32,972	49,336
Gross Unrealized Losses	4,854	702
Estimated Fair Value	1,183,253	1,176,797
Certificates of deposit held for investment
Debt Securities, Available-for-sale [Line Items]
Amortized Cost	1,992	1,992
Estimated Fair Value	$ 1,992	$ 1,992