Share-Based Compensation (Details) (USD $) In Millions, except Share data, unless otherwise specified					6 Months Ended	3 Months Ended		6 Months Ended					6 Months Ended	3 Months Ended	6 Months Ended
	Jun. 30, 2014	Dec. 31, 2013	Jun. 30, 2014 Long-Term Incentive Plan	Jun. 30, 2014 Stock Options Long-Term Incentive Plan	Jun. 30, 2014 Restricted stock units	Jun. 30, 2014 Restricted stock units Long-Term Incentive Plan	Jun. 30, 2013 Restricted stock units Long-Term Incentive Plan	Jun. 30, 2014 Restricted stock units Long-Term Incentive Plan	Jun. 30, 2013 Restricted stock units Long-Term Incentive Plan	Apr. 19, 2012 Restricted stock units Long-Term Incentive Plan	Jun. 30, 2014 Restricted stock units Long-Term Incentive Plan Personnel Accruals	Dec. 31, 2013 Restricted stock units Long-Term Incentive Plan Personnel Accruals	Jun. 30, 2014 Restricted stock units Long-Term Incentive Plan Certain executive officers	Jun. 30, 2014 Performance Unit Awards Long-Term Incentive Plan	Jun. 30, 2014 Performance Unit Awards Long-Term Incentive Plan	Jun. 30, 2014 Performance Unit Awards Long-Term Incentive Plan Personnel Accruals
Share-Based Compensation
Common stock authorized for issuance (in shares)			7,500,000	1,000,000
Offer price per share received as cash settlement on restricted stock awards vested in 2012										$ 30.00
Vesting period								3 years					1 year
Vesting rights percentage					33.33%
Number of shares, right to receive cash payment equal to fair value of which is received per unit on vesting of award								1
Unrecognized compensation cost						$ 2.9		$ 2.9
Weighted-average period for amortization of unrecognized compensation cost								9 months 18 days
Compensation expense						0.9	3.7	1.5	9.1					1.9	3.6
Liability for unvested restricted share awards	$ 43.7	$ 45.8									$ 10.3	$ 8.9				$ 7.6