Long-term Debt (Details 2) (7-5/8% Senior secured notes, USD $) In Millions, unless otherwise specified	1 Months Ended		3 Months Ended
	Mar. 31, 2011	Dec. 31, 2011	Dec. 31, 2011 Period commencing after March 31, 2015 Maximum	Dec. 31, 2011 Period commencing after March 31, 2015 Minimum	Dec. 31, 2011 On or prior to March 31, 2014	Dec. 31, 2011 Any 12-month period ending on or prior to March 31, 2014	Dec. 31, 2011 On or prior to March 31, 2015
Long-term debt
Additional borrowings	$ 400.0
Net proceeds from senior secured notes	$ 392.9
Interest rate on secured notes (as a percent)		7.625%
Debt instrument redemption price (as a percent)			103.80%	100.00%		103.00%
Percentage of the principal amount of the debt instrument which the entity may redeem with proceeds from qualified equity offerings (as a percent)			35.00%			10.00%
Redemption price of debt instrument if redeemed with proceeds from qualified equity offerings (as a percent)					107.60%
Debt instrument redemption price in addition to a make-whole premium (as a percent)							100.00%