Deferred Compensation Plan (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Dec. 31, 2012
Deferred Compensation Plan [Abstract]
RDU Plan, units authorized for issuance	28,500
RDU Plan, units outstanding	26,499
RDU Plan, compensation expense	$ 2.1	$ 2.1
RDU Plan, unrecognized compensation expense	15.6
RDU Plan, remaining term of amortization	1 year 9 months 0 days
RDU Plan, liability	$ 17.6		$ 15.5