Debt (Schedule of Line of Credit Facilities) (Details) (Details) - USD ($)		12 Months Ended
		Dec. 31, 2018	Dec. 31, 2017
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 1,260,300,000.0
Syndicated Credit Facility $850 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date		Aug. 31, 2023
Line of Credit Facility, Maximum Borrowing Capacity		$ 850,000,000
Long-term Line of Credit	[1]	$ 18,153,000	$ 95,564,000
Line of Credit Facility, Interest Rate Description		Senior unsecured long-term debt rating
Credit Facility 270 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date		Nov. 30, 2019
Credit Facility Two Thousand Nineteen [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	[2]	Mar. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	[2]	$ 191,583,000
Long-term Line of Credit	[1],[2]	$ 4,156,000	5,952,000
Line of Credit Facility, Interest Rate Description	[2]	Fixed
Credit Facility $188 Million [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Maximum Borrowing Capacity		$ 188,000,000
Long-term Line of Credit	[1]	$ 188,000,000	188,000,000
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, 2020
Line of Credit Facility, Maximum Borrowing Capacity		$ 100,000,000
Long-term Line of Credit	[1]	$ 0	75,573,000
Line of Credit Facility, Interest Rate Description		Fixed
Foreign Credit Facility December 2019 [Member]
Line of Credit Facility [Line Items]
Line of Credit Facility, Expiration Date	[2]	Dec. 31, 2019
Line of Credit Facility, Maximum Borrowing Capacity	[2]	$ 105,735,000
Long-term Line of Credit	[1],[2]	$ 105,735,000	117,135,000
Line of Credit Facility, Interest Rate Description	[2]	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	[1]	$ 61,000,000	100,000,000
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	[1]	$ 33,900,000	61,900,000
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	[1]	$ 395,000,000	$ 500,000,000
Line of Credit Facility, Interest Rate Description		Debt rating and utilization %

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

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