Debt (Details) (USD $)				1 Months Ended
	Jun. 30, 2013 Credit agreement	Dec. 31, 2010 Credit agreement	Dec. 31, 2003 Revolving Credit Facility and Senior Notes	Dec. 31, 2003 Revolving Credit Facility and Senior Notes 6.08% Senior Unsecured Notes	Dec. 31, 2003 Revolving Credit Facility and Senior Notes 6.81% Senior Unsecured Notes
Debt Instrument [Line Items]
Maximum borrowing capacity on line of credit		$ 180,000,000
Remaining amount available under the line of credit	114,100,000
Face amount on debt instruments			$ 100,000,000	$ 65,000,000	$ 35,000,000
Stated interest rate on debt instruments				6.08%	6.81%
Contractual term of debt instruments				7 years	10 years