Debt Debt (Schedule of Line of Credit Facilities) (Details) (Details) - USD ($)	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	$ 741,600,000.0
Syndicated Credit Facility $850 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Sep. 30, 2019
Line of Credit Facility, Maximum Borrowing Capacity	$ 850,000,000
Long-term Line of Credit	$ 96,095,000	$ 313,659,000
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	Jun. 30, 2019
Line of Credit Facility, Maximum Borrowing Capacity	$ 120,000,000
Long-term Line of Credit	$ 85,040,000	85,040,000
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 Two Thousand Seventeen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Nov. 30, 2017
Line of Credit Facility, Maximum Borrowing Capacity	$ 161,974,000
Long-term Line of Credit	$ 31,382,000	66,154,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility Two Thousand Eighteen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Dec. 31, 2018
Line of Credit Facility, Maximum Borrowing Capacity	$ 72,080,000
Long-term Line of Credit	$ 72,080,000	36,430,000
Line of Credit Facility, Interest Rate Description	Fixed
Credit Facility Two Thousand Nineteen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	Mar. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	$ 17,513,000
Long-term Line of Credit	$ 17,513,000	45,422,000
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, 2020
Line of Credit Facility, Maximum Borrowing Capacity	$ 100,000,000
Long-term Line of Credit	$ 70,690,000	$ 68,657,000
Line of Credit Facility, Interest Rate Description	Fixed