Debt - Narrative (Details) - Uncommitted credit lines	Dec. 31, 2018 USD ($)
Line of Credit Facility [Line Items]
Credit lines, maximum borrowing capacity	$ 319,900,000
Credit lines, remaining borrowing capacity	269,100,000
First, second, and fourth quarters
Line of Credit Facility [Line Items]
Credit lines, maximum borrowing capacity	300,000,000
Third quarter
Line of Credit Facility [Line Items]
Credit lines, maximum borrowing capacity	$ 600,000,000