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Income Taxes Dividends Paid Deduction (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Distributed paid to common stockholders $ 123,100 $ 123,004 $ 123,119
Non-dividend distributions (84,419) (89,301) (88,204)
Total dividends paid deduction attributable to earnings and profits $ 38,681 $ 33,703 $ 34,915