Debt - Revolving Credit Facility (Narrative) (Details) (Line of Credit [Member], USD $)	0 Months Ended	1 Months Ended
	Dec. 31, 2014	Apr. 30, 2011	Jun. 30, 2014	Aug. 31, 2011
April 2011 Line of Credit as Amended Second Quarter 2014 [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity			$ 200,000,000
Maturity date description		April 2016
Basis spread on variable rate	1.18%
Commitment fee percentage	0.20%
Remaining borrowing capacity	182,000,000
Amount to secure standby letters of credit	2,000,000
August 2006 Line of Credit [Member]
Line of Credit Facility [Line Items]
Maximum borrowing capacity		250,000,000
Maturity date description		August 2011
April 2011 Line of Credit [Member]
Line of Credit Facility [Line Items]
Issuance date of debt instrument		April 2011
Line of credit facility term		5 years
Maximum borrowing capacity before amendments		300,000,000
Maximum borrowing capacity				$ 450,000,000