Long-Term Debt and Credit Facility, Carrying Amount Table (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				6 Months Ended
	Jun. 30, 2013		Jun. 30, 2012		Jun. 30, 2013		Jun. 30, 2012		Dec. 31, 2012
Principal amount, carrying amount and interest expense of notes
Principal amount	$ 225.0				$ 225.0				$ 225.0
Carrying amount, net of discount	159.3				159.3				155.3
Amortization of discount and deferred financing costs					5.4		4.6
Convertible Notes
Principal amount, carrying amount and interest expense of notes
Principal amount	175.0				175.0				175.0
Less: unamortized issuance discount	(15.7)				(15.7)				(19.7)
Carrying amount, net of discount	159.3				159.3				155.3
Contractual coupon interest	1.9		1.9		3.9		3.9
Amortization of discount and deferred financing costs	2.4		2.1		4.6		4.1
Total interest expense	$ 4.3	[1]	$ 4.0	[1]	$ 8.5	[1]	$ 8.0	[1]

[1] A portion of the interest relating to the Convertible Notes is capitalized as Construction in progress.