Equity and Dividends (Schedule of Quarterly Dividends Declared and Paid) (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Fourth Quarter 2014 [Member]
Dividends Payable [Line Items]
Declaration date			Jan. 12, 2015
Dividend per share			$ 0.1400
Record date			Feb. 13, 2015
Total dividends			$ 15,089
Payment date			Mar. 06, 2015
First Quarter 2015 [Member]
Dividends Payable [Line Items]
Declaration date			May 07, 2015
Dividend per share			$ 0.1400
Record date			May 29, 2015
Total dividends			$ 14,955
Payment date			Jun. 19, 2015
Second Quarter 2015 [Member]
Dividends Payable [Line Items]
Declaration date			Aug. 06, 2015
Dividend per share			$ 0.1600
Record date			Sep. 04, 2015
Total dividends			$ 17,147
Payment date			Sep. 25, 2015
Third Quarter 2015 [Member]
Dividends Payable [Line Items]
Declaration date			Nov. 05, 2015
Dividend per share			$ 0.1600
Record date			Nov. 27, 2015
Total dividends			$ 17,149
Payment date			Dec. 18, 2015
Fourth Quarter 2015 [Member]
Dividends Payable [Line Items]
Declaration date		Jan. 15, 2016
Dividend per share		$ 0.1600
Record date		Feb. 12, 2016
Total dividends		$ 17,154
Payment date		Mar. 04, 2016
First Quarter 2016 [Member]
Dividends Payable [Line Items]
Declaration date		May 09, 2016
Dividend per share		$ 0.1600
Record date		May 24, 2016
Total dividends		$ 17,022
Payment date		Jun. 14, 2016
Second Quarter 2016 [Member]
Dividends Payable [Line Items]
Declaration date		Aug. 05, 2016
Dividend per share		$ 0.1700
Record date		Sep. 02, 2016
Total dividends		$ 18,127
Payment date		Sep. 23, 2016
Third Quarter 2016 [Member]
Dividends Payable [Line Items]
Declaration date		Nov. 08, 2016
Dividend per share		$ 0.1700
Record date		Nov. 28, 2016
Total dividends		$ 18,156
Payment date		Dec. 19, 2016
Fourth Quarter 2016 [Member]
Dividends Payable [Line Items]
Declaration date	Jan. 23, 2017
Dividend per share	$ 0.1700
Record date	Feb. 10, 2017
Total dividends	$ 18,160
Payment date	Mar. 03, 2017
First Quarter 2017 [Member]
Dividends Payable [Line Items]
Declaration date	May 04, 2017
Dividend per share	$ 0.1700
Record date	May 23, 2017
Total dividends	$ 18,188
Payment date	Jun. 13, 2017
Second Quarter 2017 [Member]
Dividends Payable [Line Items]
Declaration date	Aug. 04, 2017
Dividend per share	$ 0.1700
Record date	Aug. 25, 2017
Total dividends	$ 18,210
Payment date	Sep. 15, 2017
Third Quarter 2017 [Member]
Dividends Payable [Line Items]
Declaration date	Nov. 08, 2017
Dividend per share	$ 0.1700
Record date	Nov. 29, 2017
Total dividends	$ 18,227
Payment date	Dec. 20, 2017