Debt (Credit Facility) (Details) - USD ($)			12 Months Ended
	Sep. 16, 2014	Sep. 21, 2011	Dec. 31, 2015	Dec. 31, 2014
JP Morgan Chase Bank, N.A. and Bank of America, N.A
Line of Credit Facility [Line Items]
Amount outstanding			$ 0
Borrowings			0
Letter of Credit
Line of Credit Facility [Line Items]
Amount outstanding			4,040,000
Letter of Credit | JP Morgan Chase Bank, N.A. and Bank of America, N.A | Credit Facility 2014
Line of Credit Facility [Line Items]
Sublimit for letters of credit issued			50,000,000
Revolving Credit Facility
Line of Credit Facility [Line Items]
Senior revolving credit facility borrowing capacity			400,000,000
Senior revolving credit facility available capacity			$ 395,960,000
Revolving Credit Facility | Credit Facility 2011
Line of Credit Facility [Line Items]
Senior revolving credit facility borrowing capacity		$ 350,000,000
Term of debt instrument		4 years
Revolving Credit Facility | Credit Facility 2014
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date			Sep. 30, 2019
Revolving Credit Facility | JP Morgan Chase Bank, N.A. and Bank of America, N.A | Credit Facility 2014
Line of Credit Facility [Line Items]
Senior revolving credit facility borrowing capacity	$ 400,000,000
Term of debt instrument	5 years
Line of Credit Facility, Initiation Date	Sep. 16, 2014
Maximum borrowing capacity			$ 525,000,000
Commercial Paper | Credit Facility 2014
Line of Credit Facility [Line Items]
Amount outstanding			$ 0	$ 0