Investments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Short-term investments:
Short-term investments	$ 119.5	$ 136.7
Certificates of deposit
Short-term investments:
Investments, Cost Basis	119.5	136.7
Available-for-sale Securities, Accumulated Gross Unrealized Gain (Loss), before Tax	0.0	0.0
Assets held in rabbi trusts [Member]
Long-term investments:
Long-term investments, Cost Basis	9.0	9.3
Long-term investments, Unrealized Gain	0.6	0.5
Fair Value, Measurements, Recurring [Member] | Assets held in rabbi trusts [Member]
Long-term investments:
Fair Value of Assets Held Under Trust	9.6	9.8
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Certificates of deposit
Short-term investments:
Short-term investments	119.5	136.7
Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member] | Assets held in rabbi trusts [Member]
Long-term investments:
Fair Value of Assets Held Under Trust	$ 9.6	$ 9.8