Share-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified	1 Months Ended		0 Months Ended		1 Months Ended	3 Months Ended	12 Months Ended
	Feb. 17, 2012	Jan. 21, 2011	Jan. 15, 2010	Jan. 16, 2009	Jan. 18, 2008	Jun. 30, 2011	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Performance unit plan [Abstract]
Performance units granted	16,693	15,655	17,310	18,604	18,538
Accelerated compensation costs from amendment						$ 0.5
Compensation expense							1.4	1.2	0.8
Cash payments for prior activity							0.5
Vested awards							$ 3.1