Employee Benefits (Details) employee in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) employee	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Number of Fixed Income Funds Investing in Debt Securities Ecured by Royalty Payments [Line Items]
Number of employees sponsored by defined benefit pension plans | employee	9
Profit Sharing Bonus	$ 0	$ 5,500,000	$ 0
Employer discretionary contribution amount	10,300,000	7,200,000	4,700,000
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit obligation at beginning of year	1,233,600,000	1,202,700,000
Service cost	5,400,000	6,500,000	4,600,000
Interest cost	51,100,000	55,900,000	57,200,000
Benefits paid	(108,000,000)	(79,100,000)
Actuarial loss	63,500,000	53,400,000
Expenses paid from assets	(6,900,000)	(5,900,000)
Plan amendments	(10,700,000)	0
Other	400,000	100,000
Benefit obligation at year end	1,228,400,000	1,233,600,000	1,202,700,000
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair value of plan assets beginning of year	878,700,000	867,100,000
Actual return on plan assets	166,100,000	39,900,000
Employer contributions	68,000,000	56,500,000
Benefits paid	108,000,000	79,100,000
Expenses paid from assets	(6,900,000)	(5,900,000)
Other	400,000	200,000
Fair value of plan assets at year end	998,300,000	878,700,000	867,100,000
Funded status at year end	(230,100,000)	(354,900,000)
Pension Settlement Charge	7,600,000	0	$ 28,700,000
Noncurrent assets	3,300,000	1,200,000
Current liabilities	800,000	700,000
Noncurrent liabilities	232,600,000	355,400,000
Net actuarial loss	395,300,000	462,100,000
Net prior service credit	(10,700,000)	0
Unrecognized actuarial losses before tax	384,600,000	$ 462,100,000
Unrecognized actuarial losses net of tax	360,400,000
Actuarial loss included in accumulated other comprehensive income and expected to be recognized next year	15,000,000
Expected amortization of prior service credit	$ (400,000)