Financing Arrangements (Details) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Dec. 31, 2012	Aug. 09, 2012
Line of Credit Facility [Abstract]
Line of credit facility, maximum borrowing capacity	$ 400,000,000.0		$ 400,000,000.0			$ 200,000,000.0
Annual commitment fee on undrawn amounts of revolving credit facility			0.375%
Interest costs capitalized	1,500,000	0	2,900,000	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	200,000,000		200,000,000		165,000,000
Expiration date of revolving credit facility			Aug. 01, 2017
Interest rate at period end	2.69%		2.69%
Interest expense	$ 215,000	$ 193,000	$ 435,000	$ 386,000