Investments (Summary of Amortized Costs and Estimated Fair Values of Available-For-Sale Securities) (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	$ 1,078,474	$ 913,978
Unrealized Gains	38,261	9,551
Unrealized Losses	(922)	(5,411)
AFS securities	1,115,813	918,118
Obligations of states and political subdivisions
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	119,608	115,632
Unrealized Gains	7,627	2,779
Unrealized Losses	(115)	(328)
AFS securities	127,120	118,083
Mortgage-backed securities issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	547,396	462,593
Unrealized Gains	19,796	3,398
Unrealized Losses	(574)	(2,605)
AFS securities	566,618	463,386
Collateralized mortgage obligations issued or guaranteed by U.S. government-sponsored enterprises
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	399,937	325,200
Unrealized Gains	10,652	3,183
Unrealized Losses	(135)	(2,478)
AFS securities	410,454	325,905
Subordinated corporate bonds
Debt Securities, Available-for-sale [Abstract]
Amortized Cost	11,533	10,553
Unrealized Gains	186	191
Unrealized Losses	(98)	0
AFS securities	$ 11,621	$ 10,744