Retirement Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Operating and Administrative Expenses [Member]
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Charges to income for retirement plans	$ 2.3	$ 2.7	$ 2.5