Debt Debt (Schedule of Line of Credit Facilities) (Details) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 701,100,000.0
Syndicated Credit Facility $850 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Dec. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	$ 850,000,000
Long-term Line of Credit	$ 313,659,000	$ 204,774,000
Line of Credit Facility, Interest Rate Description	Senior unsecured long-term debt rating
Credit Facility 120 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	May 31, 2016
Line of Credit Facility, Maximum Borrowing Capacity	$ 120,000,000
Long-term Line of Credit	$ 85,040,000	80,040,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility 270 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Nov. 30, 2017
Line of Credit Facility, Maximum Borrowing Capacity	$ 270,000,000
Long-term Line of Credit	$ 270,000,000	270,000,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility 100 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Jun. 30, 2017
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000,000
Long-term Line of Credit	$ 68,657,000	81,747,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility 58 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Nov. 30, 2014
Line of Credit Facility, Maximum Borrowing Capacity	$ 129,214,000
Long-term Line of Credit	$ 66,154,000	74,623,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility 132 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Mar. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	$ 45,422,000
Long-term Line of Credit	$ 45,422,000	80,961,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility Variable Due May Two Thousand Sixteen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	May 31, 2016
Line of Credit Facility, Maximum Borrowing Capacity	$ 36,430,000
Long-term Line of Credit	$ 36,430,000	$ 28,612,000
Line of Credit Facility, Interest Rate Description	Fixed