Debt and Credit Facilities (Narrative) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020
Line of Credit Facility [Line Items]
Debt borrowing capacity	$ 500,000
Total borrowings outstanding	427,500	$ 200,000
Line of credit borrowings outstanding	330,000
Stand-by letters of credit	$ 54,900
BMO Harris Bank N.A. Term Loan Mature May 14, 2024
Schedule of Capitalization, Long-term Debt [Line Items]
Fixed rate term loan maturity	May 14, 2024
Fixed rate term loan face amount	$ 100,000
Fixed rate term loan current outstanding amount	$ 97,500
Fixed rate term loan interest rate	1.28%
Periodic payment, principal	$ 1,250
Wells Fargo Bank, N.A. Mature May 14, 2024
Line of Credit Facility [Line Items]
Total of committed credit facilities with banks	$ 300,000
Committed credit facilities maturity	May 14, 2024
Line of credit borrowings outstanding	$ 75,000
Interest rate swap facility, amount	$ 75,000
Line of credit facility, interest rate	0.78%
Interest rate swap facility, fixed interest	2.32%
Interest rate swap facility, maturity date	May 14, 2024
BMO Harris Bank N.A. Mature May 14, 2024
Line of Credit Facility [Line Items]
Debt borrowing capacity	$ 300,000	$ 200,000
Total of committed credit facilities with banks	$ 200,000
Committed credit facilities maturity	May 14, 2024
Line of credit borrowings outstanding	$ 75,000
Interest rate swap facility, amount	$ 75,000
Line of credit facility, interest rate	0.80%
Interest rate swap facility, fixed interest	2.36%
Interest rate swap facility, maturity date	May 14, 2024
Additional Outstanding Debt Under Credit Facilities
Line of Credit Facility [Line Items]
Line of credit borrowings outstanding	$ 180,000
Line of credit facility, interest rate	0.78%