Debt (Aggregate Borrowing Information) (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Unused credit lines	$ 764,500,000	$ 530,900,000
Financial Services [Member]
Line of Credit Facility [Line Items]
Available credit lines	475,000,000	360,000,000
Unused credit lines	$ 37,196,000	$ 28,379,000
Weighted-average interest rate	3.55%	2.89%