Debt (Details) (USD $)							0 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Level 2	Dec. 31, 2012 Level 2	Mar. 31, 2013 Unsecured private placement notes	Dec. 31, 2012 Unsecured private placement notes	Apr. 09, 2012 4.00% unsecured senior notes	Apr. 09, 2010 5.69% unsecured senior notes	Apr. 04, 2011 4.68% unsecured senior notes	Apr. 09, 2010 Private shelf facility	Aug. 22, 2011 Private shelf facility	Mar. 31, 2013 Revolving-credit agreement with financial institutions	Dec. 31, 2012 Revolving-credit agreement with financial institutions	Jun. 23, 2010 Revolving-credit agreement with financial institutions item	Mar. 31, 2013 Commercial paper placement agreement	Dec. 31, 2012 Commercial paper placement agreement
Debt
Total long-term debt	$ 220,500,000	$ 196,200,000			$ 150,000,000	$ 150,000,000						$ 59,000,000	$ 5,500,000		$ 11,500,000	$ 40,700,000
Interest rate (as a percent)							4.00%	5.69%	4.68%
Issuance of debt							50,000,000	50,000,000	50,000,000
Term of private shelf facility										3 years
Maximum borrowing capacity										125,000,000	150,000,000			125,000,000
Estimated fair value of long-term debt	233,000,000	209,000,000	221,000,000	196,000,000
Outstanding borrowings					150,000,000
Total amount available for borrowing					$ 200,000,000
Number of financial institutions														5