Incentive Plans (Schedules of Executive Management Share-based Compensation Activity) (Details) - Performance Shares - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Number of Shares [Roll Forward]
Shares potentially payable, beginning balance	81,712	112,465	157,405
Forfeitures (in shares)	0	0	0
Payments (in shares)	(20,680)	(45,689)	(79,762)
Shares accrued based on years performance	36,808	14,936	34,822
Shares potentially payable, ending balance	97,840	81,712	112,465
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Nonvested, Weighted Average Grant Date Fair Value [Abstract]
Shares potentially payable, beginning balance	$ 65.06	$ 49.96	$ 40.30
Forfeitures (in dollars per share)	0		0
Payments (in dollars per share)	47.35	36.74	39.90
Shares accrued based on years performance	73.43	92.12	70.59
Shares potentially payable, ending balance	$ 71.95	$ 65.06	$ 49.96
Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Aggregate Intrinsic Value [Abstract]
Shares potentially payable	$ 5,316	$ 5,619	$ 6,343
Shares potentially payable	$ 7,040	$ 5,316	$ 5,619