Employee Benefit Plans, 401(k) Plan and Other Retirement Benefits (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Other Retirement Benefits [Abstract]
Supplemental retirement benefits for retired executives under other retirement benefits plan	$ 2.4	$ 2.6
Expense related to other retirement benefits plan	0.1	0.2	$ 0.3
401(K) Plan [Member]
Defined Contribution Plan Disclosure [Line Items]
Employer contribution to plan	$ 2.8	$ 2.7	$ 2.5