Debt - Credit Facility (Details) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018
Term Loan | Domestic Line of Credit
Line of Credit Facility [Line Items]
Amounts drawn on loans	$ 276,300,000
Credit Facility | Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	400,000,000
Credit Facility | Medium-term Notes
Line of Credit Facility [Line Items]
Maximum borrowing capacity	285,000,000
Amounts drawn on loans	281,400,000
Credit Facility | Medium-term Notes | Domestic Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	279,800,000
Credit Facility | Medium-term Notes | Foreign Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	5,200,000
Amounts drawn on loans	5,200,000
Credit Facility | Delayed Draw Term Loan
Line of Credit Facility [Line Items]
Maximum borrowing capacity	30,000,000
Credit Facility | Revolving Credit Facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	30,000,000
Credit Facility | Revolving Credit Facility | Domestic Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	29,000,000
Credit Facility | Revolving Credit Facility | Foreign Line of Credit
Line of Credit Facility [Line Items]
Maximum borrowing capacity	1,000,000
Credit Facility | Uncommitted Accordion Debt
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 55,000,000
Credit Agreement
Line of Credit Facility [Line Items]
Cash interest costs	6.50%	6.60%