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Income Taxes - Tax Status of Dividends (Details) - $ / shares
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Schedule of Tax Status of Dividends [Line Items]      
Document Fiscal Year Focus 2017    
Ordinary income 86.00% 53.00% 71.00%
Capital gain 10.00% 8.00% 5.00%
Return of capital 4.00% 39.00% 19.00%
Allocation of Dividends, Qualified Dividend Income 0.00% 0.00% 5.00%
Parent Company [Member]      
Schedule of Tax Status of Dividends [Line Items]      
Dividend per share $ 2.10 $ 2.00 $ 1.94