Debt (Credit Facility) (Details) - USD ($)			6 Months Ended
	Sep. 16, 2014	Sep. 21, 2011	Jun. 30, 2016	Dec. 31, 2015
Credit Facility 2014
Line of Credit Facility [Line Items]
Amount outstanding			$ 0
Borrowings			0
Revolving Credit Facility
Line of Credit Facility [Line Items]
Credit facility borrowing capacity			400,000,000
Credit facility available capacity			395,960,000
Revolving Credit Facility | Credit Facility 2014
Line of Credit Facility [Line Items]
Credit facility borrowing capacity	$ 400,000,000
Term of debt instrument	5 years
Maximum borrowing capacity	$ 525,000,000
Revolving Credit Facility | Credit Facility 2011
Line of Credit Facility [Line Items]
Credit facility borrowing capacity		$ 350,000,000
Term of debt instrument		4 years
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding			4,040,000
Letter of Credit | Credit Facility 2014
Line of Credit Facility [Line Items]
Credit facility borrowing capacity	$ 50,000,000
Commercial Paper | Credit Facility 2014
Line of Credit Facility [Line Items]
Amount outstanding			$ 0	$ 0