Retirement Plans (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015	Dec. 31, 2015
Defined Contribution Plans
Defined contribution plan expense recognized	$ 3.1	$ 2.9	$ 7.2	$ 6.5
Maximum percentage of employee compensation match by employer to employee stock ownership plan			4.00%
Maximum percentage of employer match in the form of common stock			50.00%
Shares held by the ESOP plan	1.3		1.3		1.3
Defined Benefit Plans
Components of net periodic benefit cost
Service cost	$ 0.9	0.9	$ 1.7	1.9
Interest cost	1.4	1.8	2.7	3.5
Expected return on plan assets	(2.6)	(2.6)	(5.1)	(5.2)
Amortization of unrecognized loss	0.6	1.3	1.2	2.6
Net periodic benefit cost	$ 0.3	$ 1.4	$ 0.5	$ 2.8