Investments (Details) - USD ($) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2017	Dec. 31, 2016
Short-term investments:
Short-term investments	$ 76.9	$ 64.1
Long-term investments:
Assets held in a rabbi trust	93.7	94.7
Certificates of deposit
Short-term investments:
Investments, Cost Basis	76.9	64.1
Short-term investments, unrealized gain	0.0	0.0
Assets held in rabbi trusts [Member]
Long-term investments:
Assets held in a rabbi trust	7.8	7.9
Long-term investments, unrealized gain	1.1	0.6
Short-term investments | Certificates of deposit | Fair Value, Measurements, Recurring [Member]
Short-term investments:
Fair value of assets held under trust	76.9	64.1
Short-term investments | Certificates of deposit | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Short-term investments:
Fair value of assets held under trust	76.9	64.1
Other liabilities | Fair Value, Inputs, Level 1 [Member] | Fair Value, Measurements, Recurring [Member]
Long-term investments:
Fair Value Of Assets Held Under Trust	$ 8.9	$ 8.5