Shareholders' Equity IPO- and secondary-offering related expenses (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended				12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2014		Dec. 31, 2013
Class of Stock [Line Items]
Secondary-offering related expenses	$ 0.2	$ 0.3	$ 0.5	$ 0.4	$ 1.4	[1]	$ 0.6
Net proceeds used for termination of management services agreement					0	[2]	24.4	[2]
Other IPO Related Expenses							2.4	[1]
IPO and Secondary-offering related expenses							75.0
Accelerated share based compensation expense and related employer payroll taxes					0	[3]	40.7	[3]
Charge for payment of RDU Plan cash retention pool					0	[4]	7.5	[4]
Net proceeds used for termination of Management Services Agreement							$ 24.4

[1] Other expenses include secondary-offering expenses of $1.4 million and $0.6 million for the years ended December 31, 2014 and 2013, respectively.

[2] Represents the payment of a termination fee to affiliates of the Sponsors in connection with the termination of the management services agreement with such entities.

[3] See Note 10 for additional discussion of the impact of the IPO on the Company's equity awards.

[4] See Note 12 for additional discussion of this transaction.