Debt - Revolving Credit Facility (Narrative) (Details) - USD ($)	1 Months Ended
	Aug. 31, 2015	Dec. 31, 2017
Unsecured revolving credit facility
Line of Credit Facility [Line Items]
Line of credit facility, term	5 years
Maximum borrowing capacity	$ 200,000,000
Basis spread on variable rate	1.25%
Commitment fee (as a percent)	0.175%
Available borrowings		$ 139,600,000
Unsecured revolving credit facility | Letters of credit
Line of Credit Facility [Line Items]
Letters of credit		$ 10,400,000
Previous revolving credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 200,000,000
Farm credit facility
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000