Retirement Plans - Net Periodic Benefit Cost and General Information (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net asset (liability)
Long-term liabilities	$ (26.6)	$ (27.1)
Defined Contribution Plans
Defined contribution plan expense recognized	$ 14.8	$ 13.3	$ 12.2
Maximum company match (as a percent)	4.00%
Maximum percentage of employee compensation match by employer to employee stock ownership plan	50.00%
Shares held by the ESOP plan (in shares)	1.1	1.2	1.3
Defined Benefit Plans
Defined Benefit Plans
Company's contribution to pension plan	$ 0.0	$ 0.0
Net asset (liability)
Long-term assets	5.2	12.4
Current liabilities	(1.4)	(1.4)
Long-term liabilities	(20.9)	(20.3)
Net pension asset (liability)	(17.1)	(9.3)
Components of net periodic benefit cost
Service cost	2.6	3.4	$ 3.7
Interest cost	5.3	5.4	7.1
Expected return on plan assets	(10.2)	(10.1)	(10.2)
Amortization of unrecognized loss	2.3	2.1	4.1
Amortization of prior service credit	0.2	0.2	0.2
Net periodic benefit cost	$ 0.2	$ 1.0	$ 4.9
Weighted-average assumptions for benefit obligations
Discount rate (as a percent)	3.49%	3.86%
Rate of compensation increase (as a percent)	3.81%	3.82%
Weighted-average assumptions for net periodic benefit cost
Discount rate (as a percent)	3.91%	4.35%	3.87%
Rate of compensation increase (as a percent)	3.82%	4.29%	4.29%
Expected long-term return on plan assets (as a percent)	6.30%	6.20%	6.30%
Estimated future benefit payments
2018	$ 13.7
2019	14.2
2020	14.4
2021	14.1
2022	14.2
2023-2027	69.2
Executive supplemental and director defined benefit pension plans
Defined Benefit Plans
Company's contribution to pension plan	1.4	$ 1.0
Expected employer contributions in next fiscal year	$ 1.4