Benefit Plans (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2015	Mar. 31, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Components of net periodic benefit cost
Contributions to qualified and non qualified pension plans				$ 12.9	$ 8.7
Qualified Pension Plan [Member]
Components of net periodic benefit cost
Contributions to qualified and non qualified pension plans		$ 10.0
Pension Plan [Member]
Components of net periodic benefit cost
Service cost	$ 0.9		$ 0.7	2.7	2.2
Interest cost	5.9		5.8	17.8	17.3
Expected return on plan assets	(6.7)		(6.5)	(20.2)	(19.4)
Amortization of prior service benefit	0.0		0.0	0.0	(0.1)
Recognized net actuarial loss	1.7		0.8	5.0	2.3
Net periodic pension benefit cost	1.8		0.8	5.3	2.3
Other Benefits [Member]
Components of net periodic benefit cost
Service cost	0.0		0.0	0.0	0.0
Interest cost	0.1		0.2	0.4	0.5
Expected return on plan assets	0.0		0.0	0.0	0.0
Amortization of prior service benefit	0.0		(0.1)	(0.1)	(0.2)
Recognized net actuarial loss	0.1		0.1	0.3	0.2
Net periodic pension benefit cost	$ 0.2		$ 0.2	$ 0.6	$ 0.5