Financing Financing (Short-Term Debt) (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2017	Dec. 31, 2018
Short-term Debt [Line Items]
Long-term debt	$ 940,200,000		$ 942,300,000
Short-term borrowings and current maturities of long-term debt	63,100,000		6,900,000
Syndicated Loan Facility [Member]
Short-term Debt [Line Items]
Borrowings outstanding	5,900,000		5,300,000
Building Loan Facility [Member]
Short-term Debt [Line Items]
Borrowings outstanding	1,700,000		1,600,000
Line of Credit [Member]
Short-term Debt [Line Items]
Amortization of Debt Issuance Costs	1,600,000	$ 1,600,000
Commercial Paper [Member] | Line of Credit [Member] | CP Program [Member]
Short-term Debt [Line Items]
Maximum borrowing capacity	550,000,000		0
Borrowings outstanding	55,500,000		0
Revolving Credit Facility [Member] | Line of Credit [Member] | 2017 Facility [Member]
Short-term Debt [Line Items]
Maximum borrowing capacity	$ 550,000,000		$ 0