Retirement Plans (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014	Dec. 31, 2015	Dec. 31, 2014
Defined Contribution Plans
Defined contribution plan expense recognized	$ 2.9	$ 2.8	$ 6.5	$ 5.6
Maximum percentage of employee compensation match by employer to employee stock ownership plan			4.00%
Maximum percentage of employee compensation match by employer to non union employee stock ownership plan			50.00%
Shares held by the ESOP plan	1.4		1.4			1.4
Defined Benefit Plans
Components of net periodic benefit cost
Service cost	$ 0.9	0.9	$ 1.9	1.8
Interest cost	1.8	2.0	3.5	3.9
Expected return on plan assets	(2.6)	(2.7)	(5.2)	(5.4)
Amortization of unrecognized loss	1.3	1.0	2.6	2.1
Net periodic benefit cost	$ 1.4	$ 1.2	2.8	$ 2.4
Company's contribution to pension plan			$ 0.0
Executive supplemental and director defined benefit pension plans | Forecast
Components of net periodic benefit cost
Participant benefit payments					$ 1.0
Minimum | Defined Benefit Plans | Forecast
Components of net periodic benefit cost
Company's required contribution to pension plan					$ 0.0