Debt Debt (Schedule of Line of Credit Facilities) (Details) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2017	Dec. 31, 2016
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,266,500,000.0
Credit Facility Two Thousand Seventeen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 180,403,000
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	[2]	$ 96,564,000	$ 96,095,000
Line of Credit Facility, Interest Rate Description		Senior unsecured long-term debt rating
Foreign Credit Facility December 2019 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	[1]	Dec. 31, 2019
Long-term Line of Credit	[1],[2]	$ 117,135,000	72,080,000
Line of Credit Facility, Interest Rate Description	[1]	Fixed
Credit Facility Two Thousand Eighteen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	[1]	Nov. 30, 2018
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 117,135,000
Long-term Line of Credit	[1],[2]	$ 8,638,000	31,382,000
Line of Credit Facility, Interest Rate Description	[1]	Fixed
Credit Facility Two Thousand Nineteen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	[1]	Mar. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	[1]	$ 5,952,000
Long-term Line of Credit	[1],[2]	$ 5,952,000	17,513,000
Line of Credit Facility, Interest Rate Description	[1]	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	[2]	$ 75,573,000	70,690,000
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		Oct. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity		$ 188,000,000
Long-term Line of Credit	[2]	$ 188,000,000	270,000,000
Line of Credit Facility, Interest Rate Description		Fixed
Credit Facility $100 Million 2021 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date		May 31, 2021
Line of Credit Facility, Maximum Borrowing Capacity		$ 100,000,000
Long-term Line of Credit	[2]	$ 100,000,000	0
Line of Credit Facility, Interest Rate Description		Fixed
Syndicated Credit Facility $75 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date		Jun. 30, 2021
Line of Credit Facility, Maximum Borrowing Capacity		$ 75,000,000
Long-term Line of Credit	[2]	$ 61,900,000	85,040,000
Line of Credit Facility, Interest Rate Description		Fixed
Syndicated Credit Facility $500 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date		May 31, 2022
Line of Credit Facility, Maximum Borrowing Capacity		$ 500,000,000
Long-term Line of Credit	[2]	$ 500,000,000	$ 0
Line of Credit Facility, Interest Rate Description		Fixed

[1] Foreign currency denominated facility, amounts presented are in U.S. dollars.

[2] Represents issued but undrawn letters of credit. There was no cash borrowed for the periods presented.