Debt - Revolving Credit Facility (Narrative) (Details) - USD ($)	1 Months Ended
	Aug. 31, 2015	Dec. 31, 2016
Unsecured Revolving Credit Agreement Expiring 2020
Line of Credit Facility [Line Items]
Line of credit facility term, years	5 years
Maximum borrowing capacity	$ 200,000,000
Basis spread on variable rate, percentage	1.25%
Commitment fee percentage	0.175%
Remaining borrowing capacity		$ 169,600,000
Unsecured Revolving Credit Agreement Expiring 2020 | Standby Letters of Credit
Line of Credit Facility [Line Items]
Letters of credit		$ 5,400,000
Revolving Credit Facility Due 2016
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 200,000,000
Revolving Credit Facility Due December 2019
Line of Credit Facility [Line Items]
Maximum borrowing capacity	$ 100,000,000