Long-Term Debt and Credit Facility, Carrying Amount Table (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014		Dec. 31, 2013		Dec. 31, 2012
Principal amount, carrying amount and interest expense of notes
Principal amount	$ 225.0		$ 225.0
Carrying amount, net of discount	0		163.5
Convertible Notes
Principal amount, carrying amount and interest expense of notes
Principal amount	175.0		175.0
Less: unamortized issuance discount	(2.5)		(11.5)
Carrying amount, net of discount	172.5		163.5
Contractual coupon interest	7.9		7.9		7.9
Amortization of discount	9.1		8.2		7.3
Amortization of deferred financing costs	1.1		1.2		1.2
Total interest expense	$ 18.1	[1]	$ 17.3	[1]	$ 16.4	[1]

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