Deferred Compensation Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011	Apr. 30, 2012	Jan. 31, 2012	Dec. 31, 2011
Deferred Compensation Plan [Abstract]
RDU Plan, units authorized for issuance	28,500		28,500
RDU Plan, units granted			25,774
RDU Plan, compensation expense	$ 2.1	$ 2.0	$ 4.2	$ 4.0
RDU Plan, unrecognized compensation expense	21.9		21.9
RDU Plan, remaining term of amortization			2 years 6 months 0 days
RDU Plan, distributions paid					0.9	5.6
RDU Plan, liability	$ 12.9		$ 12.9				$ 15.2