Financing Arrangements (Details) (USD $)	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012	Aug. 09, 2012
Line of Credit Facility [Abstract]
Line of credit facility, maximum borrowing capacity	$ 400,000,000.0			$ 200,000,000.0
Annual commitment fee on undrawn amounts of revolving credit facility	0.375%
Interest costs capitalized	1,400,000	0
Commitment fee percentage, rate spread over bank base rate	1.50%
Commitment fee percentage, rate spread over LIBOR	2.50%
Revolving credit facility, amount outstanding	165,000,000		165,000,000
Expiration date of revolving credit facility	Aug. 01, 2017
Interest rate at period end	2.70%
Interest expense	$ 220,000	$ 193,000