Benefit Plans (Details Textual) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Retirement Benefits [Abstract]
Employer discretionary contribution amount	$ 4.9	$ 4.4	$ 3.8
Number of non-contributory defined benefit pension plans | plan	2
Period of average compensation for pension benefit computation	5 years
Age limit for a lump sum payment	55 years
Deferred compensation arrangement with Individual, recorded liability	$ 8.0
Long-term portion of the pension liability	32.4	28.3
AOCI net prior service cost	5.1
Future amortization of prior service cost	$ 0.5
Retirement age	65 years
Rabbi trust balance	$ 21.0
Expense related to contributions	0.1
Total deferred compensation liability	21.0	16.0
Deferred compensation liability, current	$ 1.4	$ 0.8