Employee Benefit Plans - Benefit Obligation, Plan Assets, and Funded Status (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Change in Benefit Obligation [Roll Forward]
Benefit obligation at end of year		$ 175.4
Change in Plan Assets [Roll Forward]
Fair value of plan assets at end of year	160.8	153.4
Pension Benefits
Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	175.4	189.7
Service cost	2.6	2.6	2.4
Interest cost	8.3	7.6	8.2
Plan participants' contributions	0	0
Actuarial (gain) loss	29.7	(13.2)
Benefits paid	(11.6)	(11.1)
Special or contractual termination benefits	0	0
Curtailment	0	(0.2)
Benefit obligation at end of year	204.4	175.4	189.7
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	153.4	142.3
Actual return on plan assets	13.3	22.1
Employer contributions	5.7	0.1
Benefits paid	(11.6)	(11.1)
Fair value of plan assets at end of year	160.8	153.4	142.3
Funded Status and Recognized Liability	(43.6)	(22.0)
Accumulated benefit obligation [Abstract]
Accumulated benefit obligation	203.2	173.6
Other Post-retirement Benefits
Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	12.9	10.9
Service cost	0.1	0.1	0.1
Interest cost	0.6	0.4	0.5
Plan participants' contributions	0.8	0.9
Actuarial (gain) loss	(0.7)	3.0
Benefits paid	(1.7)	(1.8)
Special or contractual termination benefits	0	0
Curtailment	0	(0.6)
Benefit obligation at end of year	12.0	12.9	10.9
Change in Plan Assets [Roll Forward]
Fair value of plan assets at beginning of year	0	0
Actual return on plan assets	0	0
Employer contributions	0	0
Benefits paid	0	0
Fair value of plan assets at end of year	0	0	0
Funded Status and Recognized Liability	$ (12.0)	$ (12.9)