Retirement and Other Benefit Plans (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) plan	Dec. 31, 2016 USD ($) Agreement	Dec. 31, 2015 USD ($) Agreement
Retirement Benefits [Abstract]
Number of defined contribution plans maintained by the company	4	4	4
Defined contribution retirement plan cost	$ 27.6	$ 23.7	$ 20.4