Employee Benefits - Summary of SERP's and Defined Benefit Plans (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Employee Benefits
Balances in Deferred Compensation Plan	$ 16.6	$ 16.0
Value of assets for funding future payouts under the deferred compensation plan	15.8
Change in plan assets
Fair value of plan assets at beginning of year	70.2
Fair value of plan assets at end of year	73.3	70.2
Amounts recognized in the statement of financial position
Noncurrent liabilities	(89.6)	(103.8)
Supplemental Executive Retirement Plans
Change in benefit obligation
Benefit obligation at beginning of year	49.4	45.9
Service cost	1.1	1.0	$ 0.9
Interest cost	1.6	1.3
Actuarial loss (gain)	1.1	2.5
Benefits paid	(1.3)	(1.3)
Plan settlements	(3.4)
Benefit obligation at end of year	48.5	49.4	45.9
Funded status
Funded status of the plans	(48.5)	(49.4)
Items not yet recognized as component of net periodic pension expense
Unrecognized net actuarial losses	14.1	15.3
Accumulated other comprehensive loss	14.1	15.3
Amounts recognized in the statement of financial position
Current liabilities	(14.8)	(4.4)
Noncurrent liabilities	(33.7)	(45.0)
Accumulated other comprehensive loss	14.1	15.3
Net amount recognized	(34.4)	(34.1)
Accumulated benefit obligation	44.2	44.7
Components of net periodic benefit cost
Service cost	1.1	1.0	0.9
Interest cost	1.6	1.3	1.5
Curtailment/settlement expense	1.0
Prior service (credit) cost		(0.3)	(0.5)
Amortization of net loss	1.4	1.5	0.7
Net periodic benefit cost	$ 5.1	$ 3.5	$ 2.6
Weighted average assumptions to determine net cost
Discount rate (as a percent)	3.45%	3.02%	4.07%
Rate of compensation increase (as a percent)	6.00%	6.00%	6.00%
Weighted average assumptions to determine benefit obligations
Discount rate (as a percent)	3.34%	3.42%
Rate of compensation increase (as a percent)	6.00%	6.00%
Expected employer contributions during 2017	$ 14.8
Defined Benefit Plans
Change in benefit obligation
Benefit obligation at beginning of year	96.3	$ 100.5
Service cost	1.6	1.7	$ 1.3
Interest cost	3.9	3.7
Actuarial loss (gain)	0.6	(7.4)
Benefits paid	(6.1)	(3.4)
Plan amendments		1.2
Plan settlements	(0.4)
Benefit obligation at end of year	95.9	96.3	100.5
Change in plan assets
Fair value of plan assets at beginning of year	70.2	72.4
Actual return on plan assets	3.9	(0.6)
Employer contributions	5.5	1.8
Benefits paid	(6.3)	(3.4)
Fair value of plan assets at end of year	73.3	70.2	72.4
Funded status
Funded status of the plans	(22.6)	(26.1)
Items not yet recognized as component of net periodic pension expense
Unrecognized net actuarial losses	25.4	25.8
Unamortized prior service cost	2.3	2.7
Accumulated other comprehensive loss	27.7	28.5
Amounts recognized in the statement of financial position
Noncurrent liabilities	(22.6)	(26.1)
Accumulated other comprehensive loss	27.7	28.5
Net amount recognized	5.1	2.4
Accumulated benefit obligation	95.9	96.3
Information for defined benefit plans with an accumulated benefit obligation and projected benefit obligation in excess of plan assets
Accumulated benefit obligation	95.9	96.3
Projected benefit obligation	95.9	96.3
Fair value of plan assets	73.3	70.2
Components of net periodic benefit cost
Service cost	1.6	1.7	1.3
Interest cost	3.9	3.7	3.8
Expected return on plan assets	(4.6)	(5.0)	(5.1)
Curtailment/settlement expense	0.1
Prior service (credit) cost	0.3	0.2	0.2
Amortization of net loss	1.5	1.8	0.4
Net periodic benefit cost	$ 2.8	$ 2.4	$ 0.6
Weighted average assumptions to determine net cost
Discount rate (as a percent)	4.13%	3.87%	4.70%
Expected long-term rate of return on plan assets (as a percent)	6.57%	6.59%	7.22%
Weighted average assumptions to determine benefit obligations
Discount rate (as a percent)	3.93%	4.13%
Expected long-term rate of return on plan assets (as a percent)	6.57%	6.59%
Expected employer contributions during 2017	$ 3.2