Employee Benefit Plans - Net Periodic Benefit Cost (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income:
Net actuarial loss/(gain)	$ 5.1	$ 4.6	$ (0.4)
Amortization of prior service cost	0.1	0.1	0.1
Amortization of net actuarial gain	(0.4)	(0.3)	(0.2)
Total recognized in other comprehensive income	4.8	4.4	(0.5)
Pension Benefits [Member]
Net Periodic Benefit Cost:
Interest cost	1.8	1.8	1.9
Expected return on plan assets	(2.1)	(1.9)	(1.7)
Amortization of prior service credit	0	0	0
Amortization of net actuarial loss	0.4	0.3	0.2
Net periodic benefit cost	0.1	0.2	0.4
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income:
Net actuarial loss/(gain)	4.7	4.1	(0.2)
Amortization of prior service cost	0	0	0
Amortization of net actuarial gain	(0.4)	(0.3)	(0.2)
Total recognized in other comprehensive income	4.3	3.8	(0.4)
Total recognized in net periodic benefit cost and other comprehensive income	4.4	4.0	0
Other Post-retirement Benefits [Member]
Net Periodic Benefit Cost:
Interest cost	0.2	0.2	0.2
Expected return on plan assets	0	0	0
Amortization of prior service credit	(0.1)	(0.1)	(0.1)
Amortization of net actuarial loss	0	0	0
Net periodic benefit cost	0.1	0.1	0.1
Other Changes in Plan Assets and Benefit Obligations Recognized in Other Comprehensive Income:
Net actuarial loss/(gain)	0.4	0.5	(0.2)
Amortization of prior service cost	0.1	0.1	0.1
Amortization of net actuarial gain	0	0	0
Total recognized in other comprehensive income	0.5	0.6	(0.1)
Total recognized in net periodic benefit cost and other comprehensive income	$ 0.6	$ 0.7	$ 0